Investments (Short-Term and Long-Term Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 48,386	$ 49,815
Securities, Available for sale Debt Securities, Gross Unrealized Gains	214	62
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(1,778)	(2,958)
Debt Securities, Available-for-sale	46,822	46,919
Held-to-maturity securities, Amortized Cost	488	512
Securities, Held to maturity, Unrecognized Holding Gain	1	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(3)	(4)
Debt Securities, Held-to-maturity, Fair Value	486	508
Debt Securities, Available-for-Sale and Held-to-Maturity, Fair Value	47,308	47,427
Debt Securities, Available-for-Sale and Held-to-Maturity, Cumulative Unrecognized Loss	(1,781)	(2,962)
Debt Securities, Available-for-Sale and Held-to-Maturity, Cumulative Unrecognized Gain	215	62
Debt Securities, Available-for-Sale and Held-to-Maturity, Amortized Cost	48,874	50,327
Debt Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale	46,822	46,919
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	3,893	4,600
Securities, Available for sale Debt Securities, Gross Unrealized Gains	2	1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(167)	(274)
Debt Securities, Available-for-sale	3,728	4,327
Held-to-maturity securities, Amortized Cost	459	444
Securities, Held to maturity, Unrecognized Holding Gain	1	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(1)	(2)
Debt Securities, Held-to-maturity, Fair Value	459	442
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	6,747	7,357
Securities, Available for sale Debt Securities, Gross Unrealized Gains	20	2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(273)	(375)
Debt Securities, Available-for-sale	6,494	6,984
Held-to-maturity securities, Amortized Cost	26	28
Securities, Held to maturity, Unrecognized Holding Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(2)	(2)
Debt Securities, Held-to-maturity, Fair Value	24	26
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	24,731	24,391
Securities, Available for sale Debt Securities, Gross Unrealized Gains	156	56
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(578)	(1,140)
Debt Securities, Available-for-sale	24,309	23,307
Held-to-maturity securities, Amortized Cost	3	40
Securities, Held to maturity, Unrecognized Holding Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	3	40
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	10,258	10,577
Securities, Available for sale Debt Securities, Gross Unrealized Gains	25	1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(651)	(994)
Debt Securities, Available-for-sale	9,632	9,584
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	2,757	2,890
Securities, Available for sale Debt Securities, Gross Unrealized Gains	11	2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(109)	(175)
Debt Securities, Available-for-sale	$ 2,659	$ 2,717

UNITEDHEALTH GROUP INC 10-Q