v3.26.1
Fair value measurements - Investments Categorized by the Level of the Fair Value Hierarchy (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale $ 5,156.9 $ 5,168.6
Total debt securities, trading 64.8 90.3
Short-term investments 7.5 28.3
Other long-term investments 285.8 315.1
Cost and equity method investments 60.9 69.3
Total assets 5,521.2 5,616.7
Liabilities, Fair Value Disclosure [Abstract]    
Derivative liabilities 29.1 9.0
Total liabilities 29.1 9.0
Asset-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 842.9 921.1
Total debt securities, trading 5.1 5.9
Residential mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 960.0 963.1
Total debt securities, trading 27.0 45.0
Commercial mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 256.1 231.9
Total debt securities, trading 26.4 31.9
Corporate debt securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 2,237.2 2,198.2
Total debt securities, trading 3.4 3.7
U.S. government and government agency    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 840.6 835.7
Total debt securities, trading 2.9 3.8
Non-U.S. government and government agency    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 20.1 18.6
Fair Value, Inputs, Level 1, 2 and 3    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 5,156.9 5,168.6
Total debt securities, trading 64.8 90.3
Short-term investments 7.5 28.3
Other long-term investments 57.5 88.1
Derivative assets 6.2 14.4
Total assets 5,292.9 5,389.7
Fair Value, Inputs, Level 1, 2 and 3 | Asset-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 842.9 921.1
Total debt securities, trading 5.1 5.9
Fair Value, Inputs, Level 1, 2 and 3 | Residential mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 960.0 963.1
Total debt securities, trading 27.0 45.0
Fair Value, Inputs, Level 1, 2 and 3 | Commercial mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 256.1 231.9
Total debt securities, trading 26.4 31.9
Fair Value, Inputs, Level 1, 2 and 3 | Corporate debt securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 2,237.2 2,198.2
Total debt securities, trading 3.4 3.7
Fair Value, Inputs, Level 1, 2 and 3 | U.S. government and government agency    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 840.6 835.7
Total debt securities, trading 2.9 3.8
Fair Value, Inputs, Level 1, 2 and 3 | Non-U.S. government and government agency    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 20.1 18.6
Quoted prices in active markets (Level 1)    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 840.6 835.7
Total debt securities, trading 2.9 3.8
Short-term investments 7.5 28.3
Other long-term investments 0.0 0.0
Derivative assets 0.0 0.0
Total assets 851.0 867.8
Liabilities, Fair Value Disclosure [Abstract]    
Derivative liabilities 0.0 0.0
Total liabilities 0.0 0.0
Quoted prices in active markets (Level 1) | Asset-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Total debt securities, trading 0.0 0.0
Quoted prices in active markets (Level 1) | Residential mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Total debt securities, trading 0.0 0.0
Quoted prices in active markets (Level 1) | Commercial mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Total debt securities, trading 0.0 0.0
Quoted prices in active markets (Level 1) | Corporate debt securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Total debt securities, trading 0.0 0.0
Quoted prices in active markets (Level 1) | U.S. government and government agency    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 840.6 835.7
Total debt securities, trading 2.9 3.8
Quoted prices in active markets (Level 1) | Non-U.S. government and government agency    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Significant other observable inputs (Level 2)    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 4,316.3 4,332.9
Total debt securities, trading 61.9 86.5
Short-term investments 0.0 0.0
Other long-term investments 0.0 5.2
Derivative assets 0.0 0.0
Total assets 4,378.2 4,424.6
Liabilities, Fair Value Disclosure [Abstract]    
Derivative liabilities 0.0 0.0
Total liabilities 0.0 0.0
Significant other observable inputs (Level 2) | Asset-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 842.9 921.1
Total debt securities, trading 5.1 5.9
Significant other observable inputs (Level 2) | Residential mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 960.0 963.1
Total debt securities, trading 27.0 45.0
Significant other observable inputs (Level 2) | Commercial mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 256.1 231.9
Total debt securities, trading 26.4 31.9
Significant other observable inputs (Level 2) | Corporate debt securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 2,237.2 2,198.2
Total debt securities, trading 3.4 3.7
Significant other observable inputs (Level 2) | U.S. government and government agency    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Total debt securities, trading 0.0 0.0
Significant other observable inputs (Level 2) | Non-U.S. government and government agency    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 20.1 18.6
Significant unobservable inputs (Level 3)    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Total debt securities, trading 0.0 0.0
Short-term investments 0.0 0.0
Other long-term investments 57.5 82.9
Derivative assets 6.2 14.4
Total assets 63.7 97.3
Liabilities, Fair Value Disclosure [Abstract]    
Derivative liabilities 29.1 9.0
Total liabilities 29.1 9.0
Significant unobservable inputs (Level 3) | Asset-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Total debt securities, trading 0.0 0.0
Significant unobservable inputs (Level 3) | Residential mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Total debt securities, trading 0.0 0.0
Significant unobservable inputs (Level 3) | Commercial mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Total debt securities, trading 0.0 0.0
Significant unobservable inputs (Level 3) | Corporate debt securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Total debt securities, trading 0.0 0.0
Significant unobservable inputs (Level 3) | U.S. government and government agency    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Total debt securities, trading 0.0 0.0
Significant unobservable inputs (Level 3) | Non-U.S. government and government agency    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 0.0 0.0
Valued at NAV    
Assets, Fair Value Disclosure [Abstract]    
Investments in funds valued at NAV $ 167.4 $ 157.7