v3.26.3
Fair value measurements - Investments Categorized by the Level of the Fair Value Hierarchy (Details) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale $ 5,364.8 $ 5,168.6
Total debt securities, trading 55.3 90.3
Short-term investments 17.1 28.3
Other long-term investments 276.5 315.1
Cost and equity method investments 60.4 69.3
Total assets 5,716.3 5,616.7
Liabilities, Fair Value Disclosure [Abstract]    
Derivative liabilities 24.6 9.0
Total liabilities 24.6 9.0
Asset-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 770.0 921.1
Total debt securities, trading 5.0 5.9
Residential mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 1,066.6 963.1
Total debt securities, trading 24.9 45.0
Commercial mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 247.5 231.9
Total debt securities, trading 19.5 31.9
Corporate debt securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 2,380.1 2,198.2
Total debt securities, trading 2.9 3.7
U.S. government and government agency    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 880.8 835.7
Total debt securities, trading 3.0 3.8
Non-U.S. government and government agency    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 19.8 18.6
Fair Value, Inputs, Level 1, 2 and 3    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 5,364.8 5,168.6
Total debt securities, trading 55.3 90.3
Short-term investments 17.1 28.3
Other long-term investments 56.9 88.1
Derivative assets 2.6 14.4
Total assets 5,496.7 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 770.0 921.1
Total debt securities, trading 5.0 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 1,066.6 963.1
Total debt securities, trading 24.9 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 247.5 231.9
Total debt securities, trading 19.5 31.9
Fair Value, Inputs, Level 1, 2 and 3 | Corporate debt securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 2,380.1 2,198.2
Total debt securities, trading 2.9 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 880.8 835.7
Total debt securities, trading 3.0 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 19.8 18.6
Quoted prices in active markets (Level 1)    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 880.8 835.7
Total debt securities, trading 3.0 3.8
Short-term investments 17.1 28.3
Other long-term investments 0.0 0.0
Derivative assets 0.0 0.0
Total assets 900.9 867.8
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 880.8 835.7
Total debt securities, trading 3.0 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 4,484.0 4,332.9
Total debt securities, trading 52.3 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 4,536.3 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 770.0 921.1
Total debt securities, trading 5.0 5.9
Significant other observable inputs (Level 2) | Residential mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 1,066.6 963.1
Total debt securities, trading 24.9 45.0
Significant other observable inputs (Level 2) | Commercial mortgage-backed securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 247.5 231.9
Total debt securities, trading 19.5 31.9
Significant other observable inputs (Level 2) | Corporate debt securities    
Assets, Fair Value Disclosure [Abstract]    
Total debt securities, available for sale 2,380.1 2,198.2
Total debt securities, trading 2.9 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 19.8 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 56.9 82.9
Derivative assets 2.6 14.4
Total assets 59.5 97.3
Liabilities, Fair Value Disclosure [Abstract]    
Derivative liabilities 24.6 9.0
Total liabilities 24.6 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 $ 159.2 $ 157.7