Employee Benefits - Summary of Equity-Based Awards for Employees (Detail)	6 Months Ended
	Jun. 30, 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	65,112
Vested | shares	(90,952)
Forfeited/Cancelled | shares	(1,329)
Nonvested common stock awards, Ending balance | shares	175,162
Nonvested performance-based stock awards, Beginning balance | $ / shares	$ 39.09
Granted | $ / shares	57.51
Vested | $ / shares	36.10
Forfeited/Cancelled | $ / shares	44.82
Nonvested performance-based stock awards, Ending balance | $ / shares	$ 47.45
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
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
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	14,072
Vested | shares	(71,120)
Forfeited/Cancelled | shares	(1,464)
Nonvested performance-based stock awards/units, Ending balance | shares	13,488
Nonvested performance-based stock awards/units, Beginning balance | $ / shares	$ 52.16
Granted | $ / shares	55.92
Vested | $ / shares	52.15
Forfeited/Cancelled | $ / shares	52.94
Nonvested performance-based stock awards/units, Ending balance | $ / shares	$ 56.05