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Borrowing Arrangements - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]      
Proceeds from loans   $ 146,000,000 $ 88,100,000
Repayments of debt $ 196,800,000 227,500,000  
Line of credit facility, maximum borrowing capacity 400,000,000    
Term loan, net 198,626,000 198,302,000  
Commitment fee   3,700,000  
Unsecured line of credit 0 30,000,000  
Term Loans      
Debt Instrument [Line Items]      
Term loan, net $ 200,000,000    
Debt instrument, maturity date Apr. 27, 2023    
Derivative, maturity years (in years) 3 years    
Line of Credit      
Debt Instrument [Line Items]      
Proceeds from loans $ 357,800,000 $ 204,400,000  
Minimum      
Debt Instrument [Line Items]      
Interest rate during period (in percentage) 0.15%    
Maximum      
Debt Instrument [Line Items]      
Interest rate during period (in percentage) 0.35%    
LIBOR | Minimum | Term Loans      
Debt Instrument [Line Items]      
Interest rate (in percentage) 1.20% 4.80%  
LIBOR | Minimum | Line of Credit      
Debt Instrument [Line Items]      
Interest rate (in percentage) 1.10%    
LIBOR | Maximum | Term Loans      
Debt Instrument [Line Items]      
Interest rate (in percentage) 1.90%    
LIBOR | Maximum | Line of Credit      
Debt Instrument [Line Items]      
Interest rate (in percentage) 1.55%