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Treasury Stock Acquisition (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule of Treasury Stock [Line Items]      
Treasury shares at beginning of period 43,392,617 43,207,240 42,818,585
Treasury shares at end of period 43,802,611 43,392,617 43,207,240
Treasury shares at beginning of period $ 911,094 $ 907,045 $ 899,035
Treasury stock acquired, cost 7,617 4,049 8,010
Treasury shares at end of period $ 918,711 $ 911,094 $ 907,045
2013 Stock Buyback Program      
Schedule of Treasury Stock [Line Items]      
Treasury stock acquired, shares 5,503 8,488 8,618
Treasury stock acquired, cost $ 109 $ 183 $ 180
Long Term Incentive Plan      
Schedule of Treasury Stock [Line Items]      
Treasury stock acquired, shares 404,491 176,889 380,037
Treasury stock acquired, cost $ 7,508 $ 3,866 $ 7,830