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Weighted Average Shares - Calculation of Weighted Average Shares (Detail) - shares
shares in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Earnings Per Share [Abstract]      
Basic weighted-average common shares outstanding 173,287 170,676 172,592
Effect of dilutive stock options 6,264 3,468 3,390
Diluted weighted-average common and common-equivalent shares outstanding 179,551 174,144 175,982