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Stock-Based Compensation Plans - Summary of Nonvested Performance Based Share Awards and Changes During Period (Detail) - Performance-Based Restricted Share Awards [Member] - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Nonvested Restricted Share Awards      
Beginning Balance 683,667 938,863 0
Granted     978,365
Forfeited (11,604) (67,397) (39,502)
Vested (484,835) (169,567)  
Change in attainment for 2015 grants (13,592) (18,232)  
Ending Balance 173,636 683,667 938,863
Weighted-Average Grant Date Fair Value      
Beginning Balance $ 23.25 $ 23.42  
Granted     $ 23.45
Forfeited 23.84 22.34 24.24
Vested 22.82 24.41  
Change in attainment for 2015 grants 23.25 24.41  
Ending Balance $ 24.41 $ 23.25 $ 23.42