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Employee Benefits - Summary of Equity-Based Awards for Employees (Detail)
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Common Stock Awards [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Nonvested common stock awards, Beginning balance | shares 161,174
Granted | shares 101,241
Vested | shares (58,302)
Forfeited/Cancelled | shares (1,782)
Nonvested common stock awards, Ending balance | shares 202,331
Nonvested performance-based stock awards, Beginning balance $ 26.36
Granted 52.00
Vested 26.43
Forfeited/Cancelled 35.07
Nonvested performance-based stock awards, Ending balance $ 39.09
Performance Based Awards [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Granted | shares 77,254
Vested | shares 0
Nonvested performance-based stock awards/units, Ending balance | shares 77,254
Nonvested performance-based stock awards/units, Beginning balance $ 0
Granted 41.00
Vested 0
Nonvested performance-based stock awards/units, Ending balance $ 41.00
Performance Based Restricted Stock Units [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Granted | shares 74,818
Vested | shares 0
Forfeited/Cancelled | shares (2,818)
Nonvested performance-based stock awards/units, Ending balance | shares 72,000
Nonvested performance-based stock awards/units, Beginning balance $ 0
Granted 52.16
Vested 0
Forfeited/Cancelled 52.00
Nonvested performance-based stock awards/units, Ending balance $ 52.16