Investments - Category and Duration of Gross Unrealized Losses on Fixed Maturity Securities and Equity Securities (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	$ 799.7	$ 2,011.0
Fixed maturity securities, less than 12 months, unrealized losses	(20.9)	(62.4)
Fixed maturity securities, 12 months or more, fair value	1,769.5	2,137.6
Fixed maturity securities, 12 months or more, unrealized losses	(269.7)	(345.6)
Fixed maturity securities, total, fair value	2,569.2	4,148.6
Fixed maturity securities, total, unrealized losses	(290.6)	(408.0)
U.S. government and government agencies and authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	8.3	25.8
Fixed maturity securities, less than 12 months, unrealized losses	(0.2)	(0.6)
Fixed maturity securities, 12 months or more, fair value	14.5	21.4
Fixed maturity securities, 12 months or more, unrealized losses	(1.5)	(2.8)
Fixed maturity securities, total, fair value	22.8	47.2
Fixed maturity securities, total, unrealized losses	(1.7)	(3.4)
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	11.9	20.4
Fixed maturity securities, less than 12 months, unrealized losses	(0.6)	(1.5)
Fixed maturity securities, 12 months or more, fair value	57.6	66.1
Fixed maturity securities, 12 months or more, unrealized losses	(6.5)	(8.7)
Fixed maturity securities, total, fair value	69.5	86.5
Fixed maturity securities, total, unrealized losses	(7.1)	(10.2)
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	87.7	164.8
Fixed maturity securities, less than 12 months, unrealized losses	(2.0)	(10.9)
Fixed maturity securities, 12 months or more, fair value	130.4	171.3
Fixed maturity securities, 12 months or more, unrealized losses	(8.9)	(14.2)
Fixed maturity securities, total, fair value	218.1	336.1
Fixed maturity securities, total, unrealized losses	(10.9)	(25.1)
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	108.1	59.0
Fixed maturity securities, less than 12 months, unrealized losses	(4.0)	(3.5)
Fixed maturity securities, 12 months or more, fair value	86.6	87.6
Fixed maturity securities, 12 months or more, unrealized losses	(6.8)	(6.0)
Fixed maturity securities, total, fair value	194.7	146.6
Fixed maturity securities, total, unrealized losses	(10.8)	(9.5)
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	21.3	65.7
Fixed maturity securities, less than 12 months, unrealized losses	(0.2)	(1.3)
Fixed maturity securities, 12 months or more, fair value	179.8	195.8
Fixed maturity securities, 12 months or more, unrealized losses	(26.6)	(35.1)
Fixed maturity securities, total, fair value	201.1	261.5
Fixed maturity securities, total, unrealized losses	(26.8)	(36.4)
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	109.0	223.4
Fixed maturity securities, less than 12 months, unrealized losses	(2.1)	(4.8)
Fixed maturity securities, 12 months or more, fair value	206.2	209.7
Fixed maturity securities, 12 months or more, unrealized losses	(40.2)	(45.7)
Fixed maturity securities, total, fair value	315.2	433.1
Fixed maturity securities, total, unrealized losses	(42.3)	(50.5)
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	303.0	1,083.8
Fixed maturity securities, less than 12 months, unrealized losses	(9.5)	(29.9)
Fixed maturity securities, 12 months or more, fair value	772.0	954.3
Fixed maturity securities, 12 months or more, unrealized losses	(134.9)	(173.9)
Fixed maturity securities, total, fair value	1,075.0	2,038.1
Fixed maturity securities, total, unrealized losses	(144.4)	(203.8)
Foreign corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	150.4	368.1
Fixed maturity securities, less than 12 months, unrealized losses	(2.3)	(9.9)
Fixed maturity securities, 12 months or more, fair value	322.4	431.4
Fixed maturity securities, 12 months or more, unrealized losses	(44.3)	(59.2)
Fixed maturity securities, total, fair value	472.8	799.5
Fixed maturity securities, total, unrealized losses	$ (46.6)	$ (69.1)

ASSURANT, INC. 10-Q