Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale
Amortized Cost, net	$ 5,797.7	$ 5,788.6
Fair Value	6,239.3	6,345.3
FNMA
Debt Securities, Available-for-sale
Fair Value	376.7	387.1
FHLMC
Debt Securities, Available-for-sale
Fair Value	326.5	344.3
GNMA
Debt Securities, Available-for-sale
Fair Value	112.1	132.3
Fixed maturity securities
Debt Securities, Available-for-sale
Amortized Cost, net	5,797.7	5,788.6
Gross Unrealized Gains	460.2	585.1
Gross Unrealized Losses	18.6	28.4
Fair Value	6,239.3	6,345.3
Mortgage-backed securities
Debt Securities, Available-for-sale
Amortized Cost, net	612.1	605.5
Gross Unrealized Gains	51.9	79.6
Gross Unrealized Losses	1.5	0.3
Fair Value	662.5	684.8
Other, including U.S. Treasury securities
Debt Securities, Available-for-sale
Amortized Cost, net	342.5	395.0
Gross Unrealized Gains	27.7	39.2
Gross Unrealized Losses	4.3	1.0
Fair Value	365.9	433.2
Municipal bonds
Debt Securities, Available-for-sale
Amortized Cost, net	1,519.7	1,612.3
Gross Unrealized Gains	184.4	215.7
Gross Unrealized Losses	0.7	0.5
Fair Value	1,703.4	1,827.5
Foreign government bonds
Debt Securities, Available-for-sale
Amortized Cost, net	40.2	40.2
Gross Unrealized Gains	3.4	4.9
Gross Unrealized Losses	0.0	0.0
Fair Value	43.6	45.1
Corporate bonds
Debt Securities, Available-for-sale
Amortized Cost, net	2,217.7	1,905.2
Gross Unrealized Gains	176.2	221.6
Gross Unrealized Losses	5.2	3.9
Fair Value	2,388.7	2,122.9
Other asset-backed securities
Debt Securities, Available-for-sale
Amortized Cost, net	1,065.5	1,230.4
Gross Unrealized Gains	16.6	24.1
Gross Unrealized Losses	6.9	22.7
Fair Value	$ 1,075.2	$ 1,231.8

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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