Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	$ 5,992.7	$ 5,788.6
Fair Value	6,356.3	6,345.3
FNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	373.3	387.1
FHLMC
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	333.6	344.3
GNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	125.7	132.3
Total Fixed Maturity Securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	5,992.7	5,788.6
Gross Unrealized Gains	412.2	585.1
Gross Unrealized Losses	48.6	28.4
Fair Value	6,356.3	6,345.3
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	627.1	605.5
Gross Unrealized Gains	52.9	79.6
Gross Unrealized Losses	1.7	0.3
Fair Value	678.3	684.8
Other, including U.S. Treasury securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	444.2	395.0
Gross Unrealized Gains	25.4	39.2
Gross Unrealized Losses	11.3	1.0
Fair Value	458.3	433.2
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	1,609.9	1,612.3
Gross Unrealized Gains	161.7	215.7
Gross Unrealized Losses	3.5	0.5
Fair Value	1,768.1	1,827.5
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	40.1	40.2
Gross Unrealized Gains	4.4	4.9
Gross Unrealized Losses	0.0	0.0
Fair Value	44.5	45.1
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	2,063.6	1,905.2
Gross Unrealized Gains	148.6	221.6
Gross Unrealized Losses	18.0	3.9
Fair Value	2,194.2	2,122.9
Other asset-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	1,207.8	1,230.4
Gross Unrealized Gains	19.2	24.1
Gross Unrealized Losses	14.1	22.7
Fair Value	$ 1,212.9	$ 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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