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Treasury Stock Acquisition (Detail)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
USD ($)
shares
Schedule of Treasury Stock [Line Items]  
Treasury shares at beginning of period | shares 43,802,611
Treasury shares at end of period | shares 44,727,411
Treasury shares at beginning of period $ 918,711
Treasury stock acquired, cost 16,928
Treasury shares at end of period $ 935,639
Stock Buyback Program  
Schedule of Treasury Stock [Line Items]  
Treasury stock acquired, shares | shares 924,800
Treasury stock acquired, cost $ 16,928