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Stock-Based Compensation Plans - Summary of Nonvested Total Shareholder Return (TSRs) Outstanding and Changes During Period (Detail) - Total Shareholder Return [Member]
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Number of Shares at Expected Attainment  
Beginning Balance | shares 0
Granted | shares 233,077
Forfeited | shares (8,624)
Change in Attainment in 2017 | shares (80,804)
Ending Balance | shares 143,649
Weighted-Average Grant Date Fair Value  
Beginning Balance | $ / shares $ 0.00
Granted | $ / shares 24.37
Forfeited | $ / shares 24.37
Change in Attainment in 2017 | $ / shares 24.37
Ending Balance | $ / shares $ 24.37