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Long-Term Debt (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Mar. 31, 2018
Aug. 02, 2016
Debt Instrument [Line Items]      
Bank Credit Facility $ 250,000 $ 240,000  
Total Debt 636,500 626,500  
Less: Debt Origination Costs (5,243) (5,578)  
Long-term Debt 631,257 620,922  
4.500% Senior Unsecured Notes Due 2026 [Member]      
Debt Instrument [Line Items]      
Total Debt 350,000 350,000 $ 350,000
Private Placement Senior Unsecured Notes [Member]      
Debt Instrument [Line Items]      
Total Debt $ 36,500 $ 36,500