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Stock-Based Compensation Plans - Summary of Nonvested PBRSAs (Detail) - Performance-Based Restricted Share Awards [Member]
12 Months Ended
Dec. 31, 2018
$ / shares
shares
Number of Shares  
Beginning Balance | shares 173,636
Vested | shares (173,636)
Weighted-Average Grant Date Fair Value  
Beginning Balance | $ / shares $ 24.41
Vested | $ / shares $ 24.41