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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]
9 Months Ended
Sep. 30, 2018
$ / shares
shares
Nonvested Restricted Share Awards  
Beginning Balance | shares 173,636
Vested | shares (173,636)
Ending Balance | shares 0
Weighted-Average Grant Date Fair Value  
Beginning Balance | $ / shares $ 24.41
Vested | $ / shares 24.41
Ending Balance | $ / shares $ 0