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Commitments and Contingencies (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Debt Instrument [Line Items]      
Investment commitments $ 1,770.0   $ 1,690.0
Interest expense on senior notes and other borrowings 57.4 $ 60.9  
Senior notes | 3.535% Senior Notes due 2050      
Debt Instrument [Line Items]      
Senior notes, face amount $ 1,000.0    
Senior notes, stated percentage 3.635%    
Description of interest rate terms Interest will be calculated on the basis of a 360-day year of twelve 30-day months    
Maturity date of senior notes Jun. 30, 2050