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Equity Transactions - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stockholders Equity [Line Items]      
Preferred stock authorized but unissued and undesignated 10,000,000 10,000,000  
Expiration date on repurchase authorizations Dec. 31, 2018    
Aggregate cost shares of common stock $ 1,510,000 $ 15,367,000 $ 11,279,000
Common stock acquired, shares 17,000    
Maximum [Member]      
Stockholders Equity [Line Items]      
Authorization amount for repurchase of common stock $ 175,000,000    
Common Stock [Member]      
Stockholders Equity [Line Items]      
Cumulative common stock acquired, shares 3,509,000    
Since Initial Approval of Share Repurchase Plan in 2011 [Member]      
Stockholders Equity [Line Items]      
Aggregate cost shares of common stock $ 77,292,000    
Remaining authorized repurchase amount $ 97,708,000