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Summary of Significant Accounting Policies - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Jan. 01, 2018
Dec. 31, 2020
Dec. 31, 2018
Jan. 01, 2020
Dec. 31, 2019
Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Period after which financial information provided by the equity method investee was used by the entity   3 months        
Increase in retained earnings   $ 335.0 $ 192.5   $ 279.4 $ 207.0
Retained Earnings            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Increase in retained earnings   $ 76.1 49.3   67.2 $ 17.9
Impact of adoption            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Increase in retained earnings [1]         (1.3)  
Cumulative effect of change in accounting principles, net of taxes [2]     0.0      
Impact of adoption | Retained Earnings            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Increase in retained earnings $ 8.1     $ (1.3) $ (1.3) [1]  
Cumulative effect of change in accounting principles, net of taxes $ (26.7)   $ (18.6) [2]      
[1] Amount relates to the adoption of a new accounting standard for accounting for expected credit losses for assets held at amortized cost, which established allowances for such expected credit losses as of January 1, 2020. See Note 2 for additional information.
[2] Amounts relate to: (i) the requirement to recognize the changes in fair value of equity securities directly within income (resulting in a reclassification of unrealized gains as of January 1, 2018 between accumulated other comprehensive income (“AOCI”) and retained earnings); and (ii) the reclassification from AOCI to retained earnings for stranded tax effects resulting from the U.S. Tax Cuts and Jobs Act. See Note 2 for additional information.