Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	$ 5,756.9	$ 5,797.7
Gross Unrealized Gains	35.0	460.2
Gross Unrealized Losses	606.9	18.6
Fair Value	5,185.0	6,239.3
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	638.2	612.1
Gross Unrealized Gains	1.3	51.9
Gross Unrealized Losses	69.1	1.5
Fair Value	570.4	662.5
Other, including U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	410.0	342.5
Gross Unrealized Gains	0.5	27.7
Gross Unrealized Losses	67.8	4.3
Fair Value	342.7	365.9
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	1,380.9	1,519.7
Gross Unrealized Gains	16.9	184.4
Gross Unrealized Losses	128.1	0.7
Fair Value	1,269.7	1,703.4
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	35.1	40.2
Gross Unrealized Gains	0.0	3.4
Gross Unrealized Losses	1.6	0.0
Fair Value	33.5	43.6
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	2,161.2	2,217.7
Gross Unrealized Gains	12.7	176.2
Gross Unrealized Losses	272.2	5.2
Fair Value	1,901.7	2,388.7
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	1,131.5	1,065.5
Gross Unrealized Gains	3.6	16.6
Gross Unrealized Losses	68.1	6.9
Fair Value	1,067.0	1,075.2
FNMA
Debt Securities, Available-for-sale [Line Items]
Fair Value	330.8	376.7
FHLMC
Debt Securities, Available-for-sale [Line Items]
Fair Value	273.3	326.5
GNMA
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 86.2	$ 112.1