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Common Stock - Summary of Information Regarding Outstanding SARs (Detail) - Stock Appreciation Rights (SARs) [Member] - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Restricted Shares Outstanding, Beginning balance 1,493,689 4,341,740
Granted 14,228 14,400
Forfeited (83,124) (283,822)
Exercised (25,491) (2,578,629)
Restricted Shares Outstanding, Ending balance 1,399,302 1,493,689
Exercisable at end of year 165,755 75,102
Weighted-Average Grant Date Fair Value of Restricted Stock, Beginning Balance $ 28.41 $ 28.32
Granted 35.09 38.86
Forfeited 28.32 28.35
Exercised 42.61 34.72
Weighted-Average Grant Date Fair Value of Restricted Stock, Ending Balance 28.49 28.41
Average Exercise Price, Exercisable at end of year $ 28.57 $ 28.33