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Investment Securities - Rollforward of Fair Value Measurements by Level (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance $ 2,746.0 $ 2,328.7  
Net realized and unrealized gains 716.4 433.0  
Amortization/accretion (4.5) (1.6)  
Purchases 766.6 866.1  
Sales (1,428.3) (950.3)  
Effect of Kudu Transaction   70.1  
Transfers in 0.0 14.9  
Transfers out 0.0 (14.9)  
Ending balance 2,796.2 2,746.0  
Fair value measured on a recurring basis      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 2,947.2    
Ending balance 2,939.1 2,947.2  
Equity Securities      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 0.1 0.0  
Net realized and unrealized gains   0.0  
Amortization/accretion   0.0  
Purchases   0.1  
Sales   0.0  
Effect of Kudu Transaction   0.0  
Transfers in   0.0  
Transfers out   0.0  
Ending balance   0.1  
Other long-term investments | Fair value measured on a recurring basis      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 474.0    
Ending balance 614.2 474.0  
Short-term investments      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Securities, realized and unrealized gain losses 0.4 0.2  
Short-term investments | Fair value measured on a recurring basis      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Short-term investments at carrying value 142.9 201.2 $ 214.2
Level 1 Inputs      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 780.0 842.6  
Net realized and unrealized gains 436.4 163.0  
Amortization/accretion (0.1) 0.2  
Purchases 133.9 165.3  
Sales (830.4) (391.1)  
Effect of Kudu Transaction   0.0  
Transfers in 0.0 0.0  
Transfers out 0.0 0.0  
Ending balance 978.5 780.0  
Level 1 Inputs | Fair value measured on a recurring basis      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 969.4    
Ending balance 1,121.4 969.4  
Level 1 Inputs | Other long-term investments | Fair value measured on a recurring basis      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 0.0    
Ending balance 0.0 0.0  
Level 1 Inputs | Short-term investments | Fair value measured on a recurring basis      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Short-term investments at carrying value 142.9 189.4  
Level 2      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 1,109.6 1,160.5  
Net realized and unrealized gains 15.9 76.1  
Amortization/accretion (4.4) (1.8)  
Purchases 437.6 404.5  
Sales (527.8) (529.7)  
Effect of Kudu Transaction   0.0  
Transfers in 0.0 0.0  
Transfers out 0.0 0.0  
Ending balance 1,030.9 1,109.6  
Level 2 | Fair value measured on a recurring basis      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 1,121.4    
Ending balance 1,030.9 1,121.4  
Level 2 | Other long-term investments | Fair value measured on a recurring basis      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 0.0    
Ending balance 0.0 0.0  
Level 2 | Short-term investments | Fair value measured on a recurring basis      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Short-term investments at carrying value 0.0 11.8  
Level 3 | Fair value measured on a recurring basis      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 654.1    
Ending balance 614.2 654.1  
Level 3 | Equity Securities      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 0.1    
Net realized and unrealized gains 0.0    
Amortization/accretion 0.0    
Purchases 0.0    
Sales (0.1)    
Transfers in 0.0    
Transfers out 0.0    
Ending balance 0.0 0.1  
Level 3 | Other long-term investments      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 654.0 138.7  
Net realized and unrealized gains 275.6 181.2  
Amortization/accretion 0.0 0.0  
Purchases 151.5 185.4  
Sales (8.2) 0.0  
Effect of Kudu Transaction   141.8  
Transfers in 0.0 10.9  
Transfers out 0.0 (4.0)  
Ending balance 614.2 654.0  
Level 3 | Other long-term investments | Fair value measured on a recurring basis      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 474.0    
Ending balance 614.2 474.0  
Level 3 | Hedge funds and private equity funds      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Amortization/accretion   0.0  
Level 3 | Short-term investments | Fair value measured on a recurring basis      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Short-term investments at carrying value 0.0 0.0  
Other Long-term Investments Measured at NAV | Hedge funds and private equity funds      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance 202.3 186.9  
Net realized and unrealized gains (11.5) 12.7  
Amortization/accretion 0.0    
Purchases 43.6 110.8  
Sales (61.8) (29.5)  
Effect of Kudu Transaction   (71.7)  
Transfers in 0.0 4.0  
Transfers out 0.0 (10.9)  
Ending balance 172.6 202.3  
MediaAlpha      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Effect of MediaAlpha IPO 0.0    
Unrealized gain associated with unconsolidated affiliates   $ 114.7  
MediaAlpha | Level 1 Inputs      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Effect of MediaAlpha IPO 458.7    
MediaAlpha | Level 2      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Effect of MediaAlpha IPO 0.0    
MediaAlpha | Level 3 | Equity Securities      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Effect of MediaAlpha IPO 0.0    
MediaAlpha | Level 3 | Other long-term investments      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Effect of MediaAlpha IPO (458.7)    
MediaAlpha | Other Long-term Investments Measured at NAV | Hedge funds and private equity funds      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Effect of MediaAlpha IPO $ 0.0