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Stock-Based Compensation Plans - Summary of Nonvested Restricted Share Awards and Changes During Period (Detail) - Restricted Share Awards (RSAs) [Member]
9 Months Ended
Sep. 30, 2018
$ / shares
shares
Nonvested Restricted Share Awards  
Beginning Balance | shares 503,237
Vested | shares (231,473)
Forfeited | shares (47,411)
Ending Balance | shares 224,353
Weighted-Average Grant Date Fair Value  
Beginning Balance | $ / shares $ 20.63
Vested | $ / shares 21.20
Forfeited | $ / shares 19.88
Ending Balance | $ / shares $ 20.18