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Employee Benefits - Summary of Equity-Based Awards for Employees (Detail)
3 Months Ended
Mar. 31, 2018
$ / shares
shares
Common Stock Awards [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Nonvested common stock awards, Beginning balance | shares 202,331
Granted | shares 7,584
Vested | shares (9,560)
Forfeited/Cancelled | shares (287)
Nonvested common stock awards, Ending balance | shares 200,068
Nonvested performance-based stock awards, Beginning balance | $ / shares $ 39.09
Granted | $ / shares 65.60
Vested | $ / shares 52.00
Forfeited/Cancelled | $ / shares 21.79
Nonvested performance-based stock awards, Ending balance | $ / shares $ 40.91
Performance Based Awards [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Nonvested performance-based stock awards/units, Beginning balance | shares 77,254
Granted | shares 52,892
Vested | shares 0
Forfeited/Cancelled | shares (14,448)
Nonvested performance-based stock awards/units, Ending balance | shares 115,698
Nonvested performance-based stock awards/units, Beginning balance | $ / shares $ 41.00
Granted | $ / shares 65.60
Vested | $ / shares 0
Forfeited/Cancelled | $ / shares 41.00
Nonvested performance-based stock awards/units, Ending balance | $ / shares $ 52.25
Performance Based Restricted Stock Units [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Nonvested performance-based stock awards/units, Beginning balance | shares 72,000
Granted | shares 584
Vested | shares 0
Forfeited/Cancelled | shares (1,464)
Nonvested performance-based stock awards/units, Ending balance | shares 71,120
Nonvested performance-based stock awards/units, Beginning balance | $ / shares $ 52.16
Granted | $ / shares 52.80
Vested | $ / shares 0
Forfeited/Cancelled | $ / shares 52.94
Nonvested performance-based stock awards/units, Ending balance | $ / shares $ 52.15