v3.26.3
Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale [1] $ 70,515.6 $ 73,360.7
Fixed maturities, trading 1,148.7 1,243.8
Equity securities 2,574.0 2,237.3
Derivative instruments, assets 1,803.3 1,224.5
Separate account assets 194,816.2 193,622.6
Investment and universal life contracts (2,432.6) (1,410.2)
Amount measured at net asset value    
Assets (liabilities) measured at fair value on a recurring basis    
Unfunded commitments of investments measured using NAV 2.9 2.9
U.S. government and agencies    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 2,190.5 1,867.6
Non-U.S. governments    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 361.0 517.7
States and political subdivisions    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 6,783.9 7,138.7
Corporate debt securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 34,501.5 37,547.5
Residential mortgage-backed pass-through securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 3,398.8 3,805.1
Commercial mortgage-backed securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 5,638.3 5,371.6
Collateralized debt obligations    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 5,798.8 6,422.3
Other debt obligations    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 11,842.8 10,690.2
Recurring Fair Value Measurements    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 70,515.6 73,360.7
Fixed maturities, trading 1,148.7 1,243.8
Equity securities 2,574.0 2,237.3
Derivative instruments, assets 1,803.3 1,224.5
Other investments 911.3 1,112.0
Cash equivalents 2,680.7 3,236.1
Market risk benefit asset 239.2 197.1
Sub-total excluding separate account assets 79,872.8 82,611.5
Separate account assets 194,816.2 193,622.6
Total assets 274,689.0 276,234.1
Investment and universal life contracts (2,432.6) (1,410.2)
Market risk benefit liability (48.9) (66.9)
Funds withheld payable embedded derivative 3,524.6 2,633.4
Derivative liabilities (673.8) (552.1)
Other liabilities (2.7) (1.3)
Total liabilities 366.6 602.9
Net assets 275,055.6 276,837.0
Recurring Fair Value Measurements | Amount measured at net asset value    
Assets (liabilities) measured at fair value on a recurring basis    
Other investments 52.1 106.4
Sub-total excluding separate account assets 52.1 106.4
Separate account assets 7,693.9 7,613.8
Total assets 7,746.0 7,720.2
Net assets 7,746.0 7,720.2
Recurring Fair Value Measurements | Fair value hierarchy Level 1    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 1,919.8 1,543.0
Fixed maturities, trading   0.1
Equity securities 1,113.8 778.6
Other investments 355.1 332.8
Cash equivalents 361.1 183.7
Sub-total excluding separate account assets 3,749.8 2,838.2
Separate account assets 126,295.2 121,376.6
Total assets 130,045.0 124,214.8
Net assets 130,045.0 124,214.8
Recurring Fair Value Measurements | Fair value hierarchy Level 2    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 64,482.1 67,513.5
Fixed maturities, trading 703.5 786.3
Equity securities 1,460.2 1,458.7
Derivative instruments, assets 1,781.2 1,199.8
Other investments 441.8 505.6
Cash equivalents 2,319.6 3,052.4
Sub-total excluding separate account assets 71,188.4 74,516.3
Separate account assets 60,061.5 63,889.7
Total assets 131,249.9 138,406.0
Derivative liabilities (656.9) (545.3)
Other liabilities (2.7) (1.3)
Total liabilities (659.6) (546.6)
Net assets 130,590.3 137,859.4
Recurring Fair Value Measurements | Fair value hierarchy Level 3    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 4,113.7 4,304.2
Fixed maturities, trading 445.2 457.4
Derivative instruments, assets 22.1 24.7
Other investments 62.3 167.2
Market risk benefit asset 239.2 197.1
Sub-total excluding separate account assets 4,882.5 5,150.6
Separate account assets 765.6 742.5
Total assets 5,648.1 5,893.1
Investment and universal life contracts (2,432.6) (1,410.2)
Market risk benefit liability (48.9) (66.9)
Funds withheld payable embedded derivative 3,524.6 2,633.4
Derivative liabilities (16.9) (6.8)
Total liabilities 1,026.2 1,149.5
Net assets 6,674.3 7,042.6
Recurring Fair Value Measurements | U.S. government and agencies    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 2,190.5 1,867.6
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 1,895.6 1,515.2
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 294.9 352.4
Recurring Fair Value Measurements | Non-U.S. governments    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 361.0 517.7
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 361.0 517.7
Recurring Fair Value Measurements | States and political subdivisions    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 6,783.9 7,138.7
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 6,720.3 7,070.3
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 63.6 68.4
Recurring Fair Value Measurements | Corporate debt securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 34,501.5 37,547.5
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 24.2 27.8
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 31,905.3 35,042.0
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 2,572.0 2,477.7
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 3,398.8 3,805.1
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 3,398.8 3,805.1
Recurring Fair Value Measurements | Commercial mortgage-backed securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 5,638.3 5,371.6
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 5,635.7 5,369.2
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 2.4
Recurring Fair Value Measurements | Collateralized debt obligations    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 5,798.8 6,422.3
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 5,798.8 6,422.3
Recurring Fair Value Measurements | Other debt obligations    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 11,842.8 10,690.2
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 10,367.3 8,934.5
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 $ 1,475.5 $ 1,755.7
[1] See Note 4, Investments, for further details relating to the amortized cost of fixed maturities, available-for-sale.