v3.26.1
Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale [1] $ 73,986.3 $ 73,360.7
Fixed maturities, trading 1,087.7 1,243.8
Equity securities 2,603.4 2,237.3
Derivative instruments, assets 1,751.3 1,224.5
Separate account assets 202,396.0 193,622.6
Investment and universal life contracts $ (2,142.5) (1,410.2)
Fixed maturities valued using internal pricing models    
Fixed maturities classified as Level 3 assets, percent valued using internal pricing models (as a percent) 2.00%  
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 1,981.0 1,867.6
Non-U.S. governments    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 397.0 517.7
States and political subdivisions    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 7,322.0 7,138.7
Corporate debt securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 38,007.1 37,547.5
Residential mortgage-backed pass-through securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 3,662.8 3,805.1
Commercial mortgage-backed securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 5,620.6 5,371.6
Collateralized debt obligations    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 5,871.3 6,422.3
Other debt obligations    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 11,124.5 10,690.2
Recurring Fair Value Measurements    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 73,986.3 73,360.7
Fixed maturities, trading 1,087.7 1,243.8
Equity securities 2,603.4 2,237.3
Derivative instruments, assets 1,751.3 1,224.5
Other investments 1,239.3 1,112.0
Cash equivalents 3,519.8 3,236.1
Market risk benefit asset 202.8 197.1
Sub-total excluding separate account assets 84,390.6 82,611.5
Separate account assets 202,396.0 193,622.6
Total assets 286,786.6 276,234.1
Investment and universal life contracts (2,142.5) (1,410.2)
Market risk benefit liability (65.9) (66.9)
Funds withheld payable embedded derivative 2,706.4 2,633.4
Derivative liabilities (526.6) (552.1)
Other liabilities (2.8) (1.3)
Total liabilities (31.4) 602.9
Net assets 286,755.2 276,837.0
Recurring Fair Value Measurements | Amount measured at net asset value    
Assets (liabilities) measured at fair value on a recurring basis    
Other investments 56.5 106.4
Sub-total excluding separate account assets 56.5 106.4
Separate account assets 7,607.0 7,613.8
Total assets 7,663.5 7,720.2
Net assets 7,663.5 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 1,696.3 1,543.0
Fixed maturities, trading   0.1
Equity securities 1,103.4 778.6
Other investments 333.7 332.8
Cash equivalents 135.4 183.7
Sub-total excluding separate account assets 3,268.8 2,838.2
Separate account assets 129,883.1 121,376.6
Total assets 133,151.9 124,214.8
Derivative liabilities (0.3)  
Total liabilities (0.3)  
Net assets 133,151.6 124,214.8
Recurring Fair Value Measurements | Fair value hierarchy Level 2    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 68,239.2 67,513.5
Fixed maturities, trading 683.2 786.3
Equity securities 1,500.0 1,458.7
Derivative instruments, assets 1,718.1 1,199.8
Other investments 760.9 505.6
Cash equivalents 3,384.4 3,052.4
Sub-total excluding separate account assets 76,285.8 74,516.3
Separate account assets 64,145.5 63,889.7
Total assets 140,431.3 138,406.0
Derivative liabilities (516.4) (545.3)
Other liabilities (2.8) (1.3)
Total liabilities (519.2) (546.6)
Net assets 139,912.1 137,859.4
Recurring Fair Value Measurements | Fair value hierarchy Level 3    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 4,050.8 4,304.2
Fixed maturities, trading 404.5 457.4
Derivative instruments, assets 33.2 24.7
Other investments 88.2 167.2
Market risk benefit asset 202.8 197.1
Sub-total excluding separate account assets 4,779.5 5,150.6
Separate account assets 760.4 742.5
Total assets 5,539.9 5,893.1
Investment and universal life contracts (2,142.5) (1,410.2)
Market risk benefit liability (65.9) (66.9)
Funds withheld payable embedded derivative 2,706.4 2,633.4
Derivative liabilities (9.9) (6.8)
Total liabilities 488.1 1,149.5
Net assets 6,028.0 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 1,981.0 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 1,670.3 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 310.7 352.4
Recurring Fair Value Measurements | Non-U.S. governments    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 397.0 517.7
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 397.0 517.7
Recurring Fair Value Measurements | States and political subdivisions    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 7,322.0 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 7,222.2 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 99.8 68.4
Recurring Fair Value Measurements | Corporate debt securities    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 38,007.1 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 26.0 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 35,464.3 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 2,516.8 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 3,662.8 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 3,662.8 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 5,620.6 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 5,618.4 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 2.2 2.4
Recurring Fair Value Measurements | Collateralized debt obligations    
Assets (liabilities) measured at fair value on a recurring basis    
Fixed maturities, available-for-sale 5,871.3 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 5,871.3 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 11,124.5 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 9,692.5 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 $ 1,432.0 $ 1,755.7
[1] See Note 3, Investments, for further details relating to the amortized cost of fixed maturities, available-for-sale.