Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	$ 3,271,288	$ 2,874,972
Total	3,271,288	2,874,972
Total fixed maturities
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	3,022,441	2,377,862
U.S. government treasuries
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	831,225	711,103
U.S. states, territories and municipalities
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	12,890	13,231
Non-U.S. sovereign governments and supranationals
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	96,308	67,527
Corporate
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	1,480,549	1,143,060
Residential mortgage-backed securities - Agency
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	363,002	272,611
Residential mortgage-backed securities - Non-agency
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	33,759	16,754
Commercial mortgage-backed securities - Non-agency
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	53,419	39,686
Other asset-backed securities
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	151,289	113,890
Short-term investments
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	248,847	497,110
Level 1
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Total	0	0
Level 1 \| Total fixed maturities
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 1 \| U.S. government treasuries
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 1 \| U.S. states, territories and municipalities
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 1 \| Non-U.S. sovereign governments and supranationals
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 1 \| Corporate
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 1 \| Residential mortgage-backed securities - Agency
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 1 \| Residential mortgage-backed securities - Non-agency
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 1 \| Commercial mortgage-backed securities - Non-agency
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 1 \| Other asset-backed securities
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 1 \| Short-term investments
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 2
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Total	3,271,288	2,874,972
Level 2 \| Total fixed maturities
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	3,022,441	2,377,862
Level 2 \| U.S. government treasuries
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	831,225	711,103
Level 2 \| U.S. states, territories and municipalities
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	12,890	13,231
Level 2 \| Non-U.S. sovereign governments and supranationals
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	96,308	67,527
Level 2 \| Corporate
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	1,480,549	1,143,060
Level 2 \| Residential mortgage-backed securities - Agency
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	363,002	272,611
Level 2 \| Residential mortgage-backed securities - Non-agency
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	33,759	16,754
Level 2 \| Commercial mortgage-backed securities - Non-agency
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	53,419	39,686
Level 2 \| Other asset-backed securities
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	151,289	113,890
Level 2 \| Short-term investments
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	248,847	497,110
Level 3
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Total	0	0
Level 3 \| Total fixed maturities
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 3 \| U.S. government treasuries
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 3 \| U.S. states, territories and municipalities
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 3 \| Non-U.S. sovereign governments and supranationals
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 3 \| Corporate
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 3 \| Residential mortgage-backed securities - Agency
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 3 \| Residential mortgage-backed securities - Non-agency
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 3 \| Commercial mortgage-backed securities - Non-agency
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 3 \| Other asset-backed securities
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	0	0
Level 3 \| Short-term investments
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Investments, at fair value	$ 0	$ 0

Hamilton Insurance Group, Ltd. 10-Q