Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	$ 5,970.7	$ 5,797.7
Fair Value	5,987.4	6,239.3
FNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	346.8	376.7
FHLMC
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	303.7	326.5
GNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	100.7	112.1
Total
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	5,970.7	5,797.7
Gross Unrealized Gains	170.8	460.2
Gross Unrealized Losses	154.1	18.6
Fair Value	5,987.4	6,239.3
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	613.4	612.1
Gross Unrealized Gains	15.2	51.9
Gross Unrealized Losses	13.1	1.5
Fair Value	615.5	662.5
Other, including U.S. Treasury securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	372.5	342.5
Gross Unrealized Gains	11.5	27.7
Gross Unrealized Losses	20.9	4.3
Fair Value	363.1	365.9
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	1,530.9	1,519.7
Gross Unrealized Gains	72.2	184.4
Gross Unrealized Losses	23.6	0.7
Fair Value	1,579.5	1,703.4
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	41.3	40.2
Gross Unrealized Gains	1.4	3.4
Gross Unrealized Losses	0.1	0.0
Fair Value	42.6	43.6
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	2,308.1	2,217.7
Gross Unrealized Gains	63.4	176.2
Gross Unrealized Losses	75.5	5.2
Fair Value	2,296.0	2,388.7
Other asset-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	1,104.5	1,065.5
Gross Unrealized Gains	7.1	16.6
Gross Unrealized Losses	20.9	6.9
Fair Value	$ 1,090.7	$ 1,075.2

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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Amortized cost of 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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