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Share Repurchase Programs (Detail) - USD ($)
 2 Months Ended 3 Months Ended 38 Months Ended
Sep. 30, 2017
Sep. 30, 2017
Sep. 30, 2017
Aug. 31, 2017
Aug. 31, 2014
Equity Class Of Treasury Stock [Line Items]          
Stock repurchase program, authorized amount         $ 50,000,000
Purchase of common stock, shares     1,316,587    
Purchase of Treasury Stock   $ 49,875,000 $ 19,000,000    
Offering and Sale of Notes          
Equity Class Of Treasury Stock [Line Items]          
Stock repurchase program, authorized amount       $ 50,000,000  
Purchase of common stock, shares 1,414,900        
Purchase of Treasury Stock $ 49,900,000        
Program          
Equity Class Of Treasury Stock [Line Items]          
Purchase of common stock, shares   0