Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	$ 5,904.1	$ 5,797.7
Fair Value	5,272.1	6,239.3
Total
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	5,904.1	5,797.7
Gross Unrealized Gains	29.3	460.2
Gross Unrealized Losses	661.3	18.6
Fair Value	5,272.1	6,239.3
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	643.7	612.1
Gross Unrealized Gains	1.5	51.9
Gross Unrealized Losses	66.1	1.5
Fair Value	579.1	662.5
Other, including U.S. Treasury securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	408.3	342.5
Gross Unrealized Gains	0.4	27.7
Gross Unrealized Losses	66.0	4.3
Fair Value	342.7	365.9
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	1,440.9	1,519.7
Gross Unrealized Gains	14.5	184.4
Gross Unrealized Losses	147.5	0.7
Fair Value	1,307.9	1,703.4
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	37.2	40.2
Gross Unrealized Gains	0.0	3.4
Gross Unrealized Losses	1.7	0.0
Fair Value	35.5	43.6
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	2,228.8	2,217.7
Gross Unrealized Gains	10.0	176.2
Gross Unrealized Losses	311.0	5.2
Fair Value	1,927.8	2,388.7
Other asset-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	1,145.2	1,065.5
Gross Unrealized Gains	2.9	16.6
Gross Unrealized Losses	69.0	6.9
Fair Value	1,079.1	1,075.2
FNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	337.5	376.7
FHLMC
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	271.1	326.5
GNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	$ 90.2	$ 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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-Section 50
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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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