Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Amortized Cost, net	$ 5,712.7	$ 5,756.9
Fair Value	5,211.9	5,185.0
Mortgage-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	650.4	638.2
Gross Unrealized Gains	0.8	1.3
Gross Unrealized Losses	68.1	69.1
Fair Value	583.1	570.4
Other, including U.S. Treasury securities
Schedule of Investments [Line Items]
Amortized Cost, net	432.4	410.0
Gross Unrealized Gains	0.3	0.5
Gross Unrealized Losses	63.6	67.8
Fair Value	369.1	342.7
Municipal bonds
Schedule of Investments [Line Items]
Amortized Cost, net	1,342.3	1,380.9
Gross Unrealized Gains	20.2	16.9
Gross Unrealized Losses	110.0	128.1
Fair Value	1,252.5	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	1.3	1.6
Fair Value	24.8	33.5
Corporate bonds
Schedule of Investments [Line Items]
Amortized Cost, net	2,053.8	2,161.2
Gross Unrealized Gains	12.6	12.7
Gross Unrealized Losses	238.0	272.2
Fair Value	1,828.4	1,901.7
Other asset-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	1,207.7	1,131.5
Gross Unrealized Gains	2.7	3.6
Gross Unrealized Losses	56.4	68.1
Fair Value	1,154.0	1,067.0
Total
Schedule of Investments [Line Items]
Amortized Cost, net	5,712.7	5,756.9
Gross Unrealized Gains	36.6	35.0
Gross Unrealized Losses	537.4	606.9
Fair Value	5,211.9	5,185.0
FNMA
Schedule of Investments [Line Items]
Fair Value	330.3	330.8
FHLMC
Schedule of Investments [Line Items]
Fair Value	293.2	273.3
GNMA
Schedule of Investments [Line Items]
Fair Value	$ 82.4	$ 86.2