v3.25.2
Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Millions
Jun. 30, 2025
Dec. 31, 2024
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale [1] $ 70,558.4 $ 68,251.0
Fixed maturities, trading 1,235.3 1,023.3
Equity securities 1,587.6 2,295.0
Derivative instruments, assets 770.2 648.2
Separate account assets 180,728.0 173,327.1
Investment and universal life contracts $ (863.7) (578.4)
Fixed maturities valued using internal pricing models    
Fixed maturities classified as Level 3 assets, percent valued using internal pricing models (as a percent) 3.00%  
Amount measured at net asset value    
Assets (liabilities) measured at fair value on a recurring basis    
Unfunded commitments of investments measured using NAV $ 3.1 3.1
U.S. government and agencies    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 1,583.4 1,637.9
Non-U.S. governments    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 497.8 495.7
States and political subdivisions    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 6,208.6 6,076.5
Corporate debt securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 36,565.4 35,702.6
Residential mortgage-backed pass-through securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 3,705.8 3,674.2
Commercial mortgage-backed securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 5,168.5 5,188.0
Collateralized debt obligations    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 6,838.8 6,560.4
Other debt obligations    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 9,990.1 8,915.7
Recurring Fair Value Measurements    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 70,558.4 68,251.0
Fixed maturities, trading 1,235.3 1,023.3
Equity securities 1,587.6 2,295.0
Mortgage loans   140.6
Derivative instruments, assets 770.2 648.2
Other investments 1,011.5 905.6
Cash equivalents 2,202.9 2,950.5
Market risk benefit asset 180.2 199.5
Sub-total excluding separate account assets 77,546.1 76,413.7
Separate account assets 180,728.0 173,327.1
Total assets 258,274.1 249,740.8
Investment and universal life contracts (863.7) (578.4)
Market risk benefit liability (68.8) (62.1)
Funds withheld payable embedded derivative 2,825.2 3,014.5
Long-term debt   (21.8)
Derivative liabilities (578.6) (506.2)
Other liabilities (3.0) (5.2)
Total liabilities 1,311.1 1,840.8
Net assets 259,585.2 251,581.6
Recurring Fair Value Measurements | Amount measured at net asset value    
Assets (liabilities) measured at fair value on a recurring basis    
Other investments 108.3 106.7
Sub-total excluding separate account assets 108.3 106.7
Separate account assets 7,428.9 7,573.6
Total assets 7,537.2 7,680.3
Net assets 7,537.2 7,680.3
Recurring Fair Value Measurements | Fair value hierarchy Level 1    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 1,217.9 1,261.0
Fixed maturities, trading 0.9  
Equity securities 241.1 990.2
Other investments 305.3 262.2
Cash equivalents 133.0 271.6
Sub-total excluding separate account assets 1,898.2 2,785.0
Separate account assets 115,694.6 112,920.3
Total assets 117,592.8 115,705.3
Other liabilities (3.0) (5.2)
Total liabilities (3.0) (5.2)
Net assets 117,589.8 115,700.1
Recurring Fair Value Measurements | Fair value hierarchy Level 2    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 65,012.7 63,091.8
Fixed maturities, trading 742.5 460.7
Equity securities 1,346.5 1,304.8
Derivative instruments, assets 764.6 626.8
Other investments 478.6 405.5
Cash equivalents 2,069.9 2,678.9
Sub-total excluding separate account assets 70,414.8 68,568.5
Separate account assets 56,863.1 52,106.5
Total assets 127,277.9 120,675.0
Long-term debt   (21.8)
Derivative liabilities (559.9) (498.3)
Total liabilities (559.9) (520.1)
Net assets 126,718.0 120,154.9
Recurring Fair Value Measurements | Fair value hierarchy Level 3    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 4,327.8 3,898.2
Fixed maturities, trading 491.9 562.6
Mortgage loans   140.6
Derivative instruments, assets 5.6 21.4
Other investments 119.3 131.2
Market risk benefit asset 180.2 199.5
Sub-total excluding separate account assets 5,124.8 4,953.5
Separate account assets 741.4 726.7
Total assets 5,866.2 5,680.2
Investment and universal life contracts (863.7) (578.4)
Market risk benefit liability (68.8) (62.1)
Funds withheld payable embedded derivative 2,825.2 3,014.5
Derivative liabilities (18.7) (7.9)
Total liabilities 1,874.0 2,366.1
Net assets 7,740.2 8,046.3
Recurring Fair Value Measurements | U.S. government and agencies    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 1,583.4 1,637.9
Recurring Fair Value Measurements | U.S. government and agencies | Fair value hierarchy Level 1    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 1,190.5 1,231.9
Recurring Fair Value Measurements | U.S. government and agencies | Fair value hierarchy Level 2    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 392.9 406.0
Recurring Fair Value Measurements | Non-U.S. governments    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 497.8 495.7
Recurring Fair Value Measurements | Non-U.S. governments | Fair value hierarchy Level 2    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 497.8 495.7
Recurring Fair Value Measurements | States and political subdivisions    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 6,208.6 6,076.5
Recurring Fair Value Measurements | States and political subdivisions | Fair value hierarchy Level 2    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 6,140.8 6,009.7
Recurring Fair Value Measurements | States and political subdivisions | Fair value hierarchy Level 3    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 67.8 66.8
Recurring Fair Value Measurements | Corporate debt securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 36,565.4 35,702.6
Recurring Fair Value Measurements | Corporate debt securities | Fair value hierarchy Level 1    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 27.4 29.1
Recurring Fair Value Measurements | Corporate debt securities | Fair value hierarchy Level 2    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 34,094.0 33,323.1
Recurring Fair Value Measurements | Corporate debt securities | Fair value hierarchy Level 3    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 2,444.0 2,350.4
Recurring Fair Value Measurements | Residential mortgage-backed pass-through securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 3,705.8 3,674.2
Recurring Fair Value Measurements | Residential mortgage-backed pass-through securities | Fair value hierarchy Level 2    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 3,705.8 3,674.2
Recurring Fair Value Measurements | Commercial mortgage-backed securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 5,168.5 5,188.0
Recurring Fair Value Measurements | Commercial mortgage-backed securities | Fair value hierarchy Level 2    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 5,165.9 5,185.3
Recurring Fair Value Measurements | Commercial mortgage-backed securities | Fair value hierarchy Level 3    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 2.6 2.7
Recurring Fair Value Measurements | Collateralized debt obligations    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 6,838.8 6,560.4
Recurring Fair Value Measurements | Collateralized debt obligations | Fair value hierarchy Level 2    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 6,838.8 6,560.4
Recurring Fair Value Measurements | Other debt obligations    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 9,990.1 8,915.7
Recurring Fair Value Measurements | Other debt obligations | Fair value hierarchy Level 2    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 8,176.7 7,437.4
Recurring Fair Value Measurements | Other debt obligations | Fair value hierarchy Level 3    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale $ 1,813.4 $ 1,478.3
[1] See Note 4, Investments, for further details relating to the amortized cost of fixed maturities, available-for-sale.