v3.25.2
Fair value measurements - Investments Categorized by the Level of the Fair Value Hierarchy (Details) - USD ($)
$ in Millions
Jun. 30, 2025
Dec. 31, 2024
Assets    
Debt securities, available-for-sale $ 4,735.9 $ 5,131.0
Debt securities, trading 102.9 162.2
Short-term investments 54.9 95.8
Other long-term investments 320.1 316.5
Cost and equity method investments 68.8 64.7
Total assets 5,232.9 5,706.4
Liabilities    
Derivative liabilities 12.3 14.3
Total liabilities 12.3 14.3
Asset-backed securities    
Assets    
Debt securities, available-for-sale 984.4 1,149.7
Debt securities, trading 9.7 53.1
Residential mortgage-backed securities    
Assets    
Debt securities, available-for-sale 916.9 973.8
Debt securities, trading 47.0 48.7
Commercial mortgage-backed securities    
Assets    
Debt securities, available-for-sale 177.6 224.5
Debt securities, trading 38.6 51.8
Corporate debt securities    
Assets    
Debt securities, available-for-sale 1,701.3 1,899.9
Debt securities, trading 3.5 4.6
U.S. government and government agency    
Assets    
Debt securities, available-for-sale 936.2 859.0
Debt securities, trading 4.1 4.0
Non-U.S. government and government agency    
Assets    
Debt securities, available-for-sale 19.5 24.1
Fair Value, Inputs, Level 1, 2 and 3    
Assets    
Debt securities, available-for-sale 4,735.9 5,131.0
Debt securities, trading 102.9 162.2
Short-term investments 54.9 95.8
Other long-term investments 92.2 89.9
Derivative assets 19.1 0.9
Total assets 5,005.0 5,479.8
Fair Value, Inputs, Level 1, 2 and 3 | Asset-backed securities    
Assets    
Debt securities, available-for-sale 984.4 1,149.7
Debt securities, trading 9.7 53.1
Fair Value, Inputs, Level 1, 2 and 3 | Residential mortgage-backed securities    
Assets    
Debt securities, available-for-sale 916.9 973.8
Debt securities, trading 47.0 48.7
Fair Value, Inputs, Level 1, 2 and 3 | Commercial mortgage-backed securities    
Assets    
Debt securities, available-for-sale 177.6 224.5
Debt securities, trading 38.6 51.8
Fair Value, Inputs, Level 1, 2 and 3 | Corporate debt securities    
Assets    
Debt securities, available-for-sale 1,701.3 1,899.9
Debt securities, trading 3.5 4.6
Fair Value, Inputs, Level 1, 2 and 3 | U.S. government and government agency    
Assets    
Debt securities, available-for-sale 936.2 859.0
Debt securities, trading 4.1 4.0
Fair Value, Inputs, Level 1, 2 and 3 | Non-U.S. government and government agency    
Assets    
Debt securities, available-for-sale 19.5 24.1
Quoted prices in active markets (Level 1)    
Assets    
Debt securities, available-for-sale 936.2 859.0
Debt securities, trading 4.1 4.0
Short-term investments 54.9 73.6
Other long-term investments 0.0 0.3
Derivative assets 0.0 0.0
Total assets 995.2 936.9
Liabilities    
Derivative liabilities 0.0 0.0
Total liabilities 0.0 0.0
Quoted prices in active markets (Level 1) | Asset-backed securities    
Assets    
Debt securities, available-for-sale 0.0 0.0
Debt securities, trading 0.0 0.0
Quoted prices in active markets (Level 1) | Residential mortgage-backed securities    
Assets    
Debt securities, available-for-sale 0.0 0.0
Debt securities, trading 0.0 0.0
Quoted prices in active markets (Level 1) | Commercial mortgage-backed securities    
Assets    
Debt securities, available-for-sale 0.0 0.0
Debt securities, trading 0.0 0.0
Quoted prices in active markets (Level 1) | Corporate debt securities    
Assets    
Debt securities, available-for-sale 0.0 0.0
Debt securities, trading 0.0 0.0
Quoted prices in active markets (Level 1) | U.S. government and government agency    
Assets    
Debt securities, available-for-sale 936.2 859.0
Debt securities, trading 4.1 4.0
Quoted prices in active markets (Level 1) | Non-U.S. government and government agency    
Assets    
Debt securities, available-for-sale 0.0 0.0
Significant other observable inputs (Level 2)    
Assets    
Debt securities, available-for-sale 3,799.7 4,272.0
Debt securities, trading 98.8 158.2
Short-term investments 0.0 22.2
Other long-term investments 5.2 3.0
Derivative assets 0.0 0.0
Total assets 3,903.7 4,455.4
Liabilities    
Derivative liabilities 0.0 0.0
Total liabilities 0.0 0.0
Significant other observable inputs (Level 2) | Asset-backed securities    
Assets    
Debt securities, available-for-sale 984.4 1,149.7
Debt securities, trading 9.7 53.1
Significant other observable inputs (Level 2) | Residential mortgage-backed securities    
Assets    
Debt securities, available-for-sale 916.9 973.8
Debt securities, trading 47.0 48.7
Significant other observable inputs (Level 2) | Commercial mortgage-backed securities    
Assets    
Debt securities, available-for-sale 177.6 224.5
Debt securities, trading 38.6 51.8
Significant other observable inputs (Level 2) | Corporate debt securities    
Assets    
Debt securities, available-for-sale 1,701.3 1,899.9
Debt securities, trading 3.5 4.6
Significant other observable inputs (Level 2) | U.S. government and government agency    
Assets    
Debt securities, available-for-sale 0.0 0.0
Debt securities, trading 0.0 0.0
Significant other observable inputs (Level 2) | Non-U.S. government and government agency    
Assets    
Debt securities, available-for-sale 19.5 24.1
Significant unobservable inputs (Level 3)    
Assets    
Debt securities, available-for-sale 0.0 0.0
Debt securities, trading 0.0 0.0
Short-term investments 0.0 0.0
Other long-term investments 87.0 86.6
Derivative assets 19.1 0.9
Total assets 106.1 87.5
Liabilities    
Derivative liabilities 12.3 14.3
Total liabilities 12.3 14.3
Significant unobservable inputs (Level 3) | Asset-backed securities    
Assets    
Debt securities, available-for-sale 0.0 0.0
Debt securities, trading 0.0 0.0
Significant unobservable inputs (Level 3) | Residential mortgage-backed securities    
Assets    
Debt securities, available-for-sale 0.0 0.0
Debt securities, trading 0.0 0.0
Significant unobservable inputs (Level 3) | Commercial mortgage-backed securities    
Assets    
Debt securities, available-for-sale 0.0 0.0
Debt securities, trading 0.0 0.0
Significant unobservable inputs (Level 3) | Corporate debt securities    
Assets    
Debt securities, available-for-sale 0.0 0.0
Debt securities, trading 0.0 0.0
Significant unobservable inputs (Level 3) | U.S. government and government agency    
Assets    
Debt securities, available-for-sale 0.0 0.0
Debt securities, trading 0.0 0.0
Significant unobservable inputs (Level 3) | Non-U.S. government and government agency    
Assets    
Debt securities, available-for-sale 0.0 0.0
Valued at NAV    
Assets    
Investments in funds valued at NAV $ 159.1 $ 161.9