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Borrowings - Senior Credit Facility (Detail) - USD ($)
12 Months Ended
Feb. 11, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Feb. 10, 2019
Debt Instrument [Line Items]          
Borrowings under credit facilities   $ 92,700,000 $ 0 $ 250,000,000.0  
Repayments under credit facility   56,900,000 0 250,000,000.0  
Interest expense   82,100,000 82,200,000 65,500,000  
Revolving credit facililty          
Debt Instrument [Line Items]          
Borrowing capacity $ 775,000,000.0 $ 775,000,000.0     $ 750,000,000.0
Interest rate at end of period   3.01%      
Borrowings under credit facilities   $ 0 0 250,000,000  
Outstanding borrowing   0      
Repayments under credit facility       250,000,000  
Interest expense   $ 0 $ 0 $ 0  
Senior Credit Facility Term Loan | Revolving credit facililty          
Debt Instrument [Line Items]          
Borrowing capacity         $ 25,000,000.0
Repayments under credit facility $ 25,000,000        
Base rate          
Debt Instrument [Line Items]          
Applicable margin over base rate   0.50%      
LIBOR rate          
Debt Instrument [Line Items]          
Applicable margin over base rate   1.50%