Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Unrealized gains and losses on fixed maturities and equity securities
Total	$ 6,045.6	$ 5,788.6
Fair Value	6,512.0	6,345.3
FNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	394.9	387.1
FHLMC
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	337.8	344.3
GNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	118.2	132.3
Total Fixed Maturity Securities
Unrealized gains and losses on fixed maturities and equity securities
Total	6,045.6	5,788.6
Gross Unrealized Gains	485.5	585.1
Gross Unrealized Losses	19.1	28.4
Fair Value	6,512.0	6,345.3
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Total	636.2	605.5
Gross Unrealized Gains	58.3	79.6
Gross Unrealized Losses	1.0	0.3
Fair Value	693.5	684.8
Other, including U.S. Treasury securities
Unrealized gains and losses on fixed maturities and equity securities
Total	385.7	395.0
Gross Unrealized Gains	28.8	39.2
Gross Unrealized Losses	5.6	1.0
Fair Value	408.9	433.2
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Total	1,592.5	1,612.3
Gross Unrealized Gains	184.1	215.7
Gross Unrealized Losses	1.1	0.5
Fair Value	1,775.5	1,827.5
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Total	40.2	40.2
Gross Unrealized Gains	3.8	4.9
Gross Unrealized Losses	0.0	0.0
Fair Value	44.0	45.1
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Total	2,268.8	1,905.2
Gross Unrealized Gains	189.4	221.6
Gross Unrealized Losses	6.6	3.9
Fair Value	2,451.6	2,122.9
Other asset-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Total	1,122.2	1,230.4
Gross Unrealized Gains	21.1	24.1
Gross Unrealized Losses	4.8	22.7
Fair Value	$ 1,138.5	$ 1,231.8