Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	$ 6,049.4	$ 5,788.6
Fair Value	6,555.0	6,345.3
FNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	380.3	387.1
FHLMC
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	331.9	344.3
GNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	123.8	132.3
Total Fixed Maturity Securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	6,049.4	5,788.6
Gross Unrealized Gains	529.3	585.1
Gross Unrealized Losses	23.7	28.4
Fair Value	6,555.0	6,345.3
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	610.8	605.5
Gross Unrealized Gains	68.0	79.6
Gross Unrealized Losses	0.5	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	430.5	395.0
Gross Unrealized Gains	31.0	39.2
Gross Unrealized Losses	5.3	1.0
Fair Value	456.2	433.2
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	1,598.6	1,612.3
Gross Unrealized Gains	200.9	215.7
Gross Unrealized Losses	0.4	0.5
Fair Value	1,799.1	1,827.5
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	40.2	40.2
Gross Unrealized Gains	4.2	4.9
Gross Unrealized Losses	0.0	0.0
Fair Value	44.4	45.1
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	2,211.3	1,905.2
Gross Unrealized Gains	202.5	221.6
Gross Unrealized Losses	5.0	3.9
Fair Value	2,408.8	2,122.9
Other asset-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost, net	1,158.0	1,230.4
Gross Unrealized Gains	22.7	24.1
Gross Unrealized Losses	12.5	22.7
Fair Value	$ 1,168.2	$ 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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