Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Amortized Cost, net	$ 5,641.6	$ 5,756.9
Fair Value	4,961.6	5,185.0
Mortgage-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	687.2	638.2
Gross Unrealized Gains	0.5	1.3
Gross Unrealized Losses	97.2	69.1
Fair Value	590.5	570.4
Other, including U.S. Treasury securities
Schedule of Investments [Line Items]
Amortized Cost, net	449.6	410.0
Gross Unrealized Gains	0.3	0.5
Gross Unrealized Losses	85.0	67.8
Fair Value	364.9	342.7
Municipal bonds
Schedule of Investments [Line Items]
Amortized Cost, net	1,334.1	1,380.9
Gross Unrealized Gains	11.2	16.9
Gross Unrealized Losses	159.8	128.1
Fair Value	1,185.5	1,269.7
Foreign government bonds
Schedule of Investments [Line Items]
Amortized Cost, net	26.1	35.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	1.6	1.6
Fair Value	24.5	33.5
Corporate bonds
Schedule of Investments [Line Items]
Amortized Cost, net	1,982.7	2,161.2
Gross Unrealized Gains	7.2	12.7
Gross Unrealized Losses	304.6	272.2
Fair Value	1,685.3	1,901.7
Other asset-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	1,161.9	1,131.5
Gross Unrealized Gains	3.6	3.6
Gross Unrealized Losses	54.6	68.1
Fair Value	1,110.9	1,067.0
Total
Schedule of Investments [Line Items]
Amortized Cost, net	5,641.6	5,756.9
Gross Unrealized Gains	22.8	35.0
Gross Unrealized Losses	702.8	606.9
Fair Value	4,961.6	5,185.0
FNMA
Schedule of Investments [Line Items]
Fair Value	323.6	330.8
FHLMC
Schedule of Investments [Line Items]
Fair Value	305.8	273.3
GNMA
Schedule of Investments [Line Items]
Fair Value	$ 78.4	$ 86.2