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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

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