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Common Stock - Summary of Information Regarding Outstanding Restricted Shares (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Restricted Shares Outstanding, Beginning balance 4,563,983 1,969,250 1,569,141
Restricted Stock Granted, Shares 1,738,589 3,384,325 954,075
Restricted Stock Vested, Shares (1,018,206) (565,202) (405,327)
Restricted Stock Forfeited, Shares (394,688) (224,390) (148,639)
Restricted Shares Outstanding, Ending balance 4,889,678 4,563,983 1,969,250
Weighted-Average Grant Date Fair Value of Restricted Stock, Beginning Balance $ 41.10 $ 61.53 $ 73.73
Restricted Stock Granted, Weighted-Average Grant Date Fair Value 38.74 31.59 53.27
Restricted Stock Vested, Weighted-Average Grant Date Fair Value 34.84 29.32 54.30
Restricted Stock Forfeited, Weighted-Average Grant Date Fair Value 55.22 49.95 62.73
Weighted-Average Grant Date Fair Value of Restricted Stock, Ending Balance $ 37.04 $ 41.10 $ 61.53