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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

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