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Long-Term Debt - Long-Term Debt Instrument (Detail) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Jan. 15, 2015
Sep. 18, 2014
Debt Instrument [Line Items]        
Document Period End Date Jun. 30, 2018      
Total current portion of long-term debt $ 25,000 $ 120,000    
Total notes 499,022 498,943    
Long-term credit agreements 350,000 365,000    
Debt Issuance Costs, Gross (7,063) (7,802)    
Total long-term debt 841,959 856,141    
Current portion of long-term debt 25,000 120,000    
Total debt 866,959 976,141    
4.500% senior notes, Series E, quarterly interest payments, balloon due September 2018        
Debt Instrument [Line Items]        
Current portion of senior notes 0 100,000 $ 25,000  
4.200% senior notes, semi-annual interest payments, net of the unamortized discount, balloon due 2024        
Debt Instrument [Line Items]        
Total notes 499,022 498,943    
5-year term-loan facility, periodic interest and principal payments, LIBOR plus up to 1.750%, expires June 28, 2022        
Debt Instrument [Line Items]        
Current portion of loan facility 25,000 20,000    
Long-term credit agreements 350,000 365,000    
5-year revolving-loan facility, periodic interest payments, LIBOR plus up to 1.500%, plus commitment fees up to 0.250%, expires June 28, 2022        
Debt Instrument [Line Items]        
Long-term credit agreements $ 0 $ 0    
Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member] | Term Loan [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Basis Spread on Variable Rate 1.00%      
Revolving Credit Facility [Member]        
Debt Instrument [Line Items]        
Long-term credit agreements       $ 475,000
Revolving Credit Facility [Member] | Minimum [Member]        
Debt Instrument [Line Items]        
Line of Credit Facility, Commitment Fee Percentage 0.15%      
Revolving Credit Facility [Member] | Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Basis Spread on Variable Rate 0.85%