Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	$ 6,046.5	$ 5,797.7
Fair Value	5,688.0	6,239.3
Total
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	6,046.5	5,797.7
Gross Unrealized Gains	66.5	460.2
Gross Unrealized Losses	425.0	18.6
Fair Value	5,688.0	6,239.3
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	640.4	612.1
Gross Unrealized Gains	5.2	51.9
Gross Unrealized Losses	33.8	1.5
Fair Value	611.8	662.5
Other, including U.S. Treasury securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	406.7	342.5
Gross Unrealized Gains	3.9	27.7
Gross Unrealized Losses	42.0	4.3
Fair Value	368.6	365.9
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	1,476.7	1,519.7
Gross Unrealized Gains	30.4	184.4
Gross Unrealized Losses	80.8	0.7
Fair Value	1,426.3	1,703.4
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	41.2	40.2
Gross Unrealized Gains	0.7	3.4
Gross Unrealized Losses	0.5	0.0
Fair Value	41.4	43.6
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	2,342.9	2,217.7
Gross Unrealized Gains	21.8	176.2
Gross Unrealized Losses	214.2	5.2
Fair Value	2,150.5	2,388.7
Other asset-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	1,138.6	1,065.5
Gross Unrealized Gains	4.5	16.6
Gross Unrealized Losses	53.7	6.9
Fair Value	1,089.4	1,075.2
FNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	349.4	376.7
FHLMC
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	293.1	326.5
GNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	$ 96.7	$ 112.1

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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Amount, before tax, of unrealized loss 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 excluding accrued interest, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

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