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Stock-Based Compensation - Summary of Stock Awards (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Non-Vested Shares (1)    
Non-Vested Shares, Beginning Balance (in shares) 1,006,471  
Non-Vested Shares, Awarded (in shares) 374,569  
Non-Vested Shares, Vested (in shares) (236,820)  
Non-Vested Shares, Cancelled/forfeited (in shares) (338,463)  
Non-Vested Shares, Ending Balance (in shares) 805,757  
Weighted Average Grant Date Fair Value    
Weighted Average Grant Date Fair Value, Beginning Balance (in dollars per share) $ 31.69  
Weighted Average Grant Date Fair Value, Awarded (in dollars per share) 33.09  
Weighted Average Grant Date Fair Value, Vested (in dollars per share) 36.43  
Weighted Average Grant Date Fair Value, Cancelled/forfeited (in dollars per share) 31.61  
Weighted Average Grant Date Fair Value, Ending Balance (in dollars per share) $ 30.98  
Performance Shares    
Weighted Average Grant Date Fair Value    
Maximum shares that could vest (percent) 200.00%  
Maximum number that could vest (in shares) 1,038,272 1,340,375