Employee Benefits - Summary of Equity-Based Awards for Employees (Detail)	9 Months Ended
	Sep. 30, 2020 $ / shares shares
Common Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested common stock awards, Beginning balance | shares	152,882
Granted | shares	159,876
Vested | shares	(90,870)
Forfeited/Cancelled | shares	(4,023)
Nonvested common stock awards, Ending balance | shares	217,865
Nonvested performance-based stock awards, Beginning balance | $ / shares	$ 52.93
Granted | $ / shares	40.02
Vested | $ / shares	49.69
Forfeited/Cancelled | $ / shares	46.76
Nonvested performance-based stock awards, Ending balance | $ / shares	$ 44.93
Performance Based Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested performance-based stock awards/units, Beginning balance | shares	160,289
Granted | shares	57,450
Vested | shares	(54,502)
Forfeited/Cancelled | shares	0
Nonvested performance-based stock awards/units, Ending balance | shares	163,237
Nonvested performance-based stock awards/units, Beginning balance | $ / shares	$ 50.49
Granted | $ / shares	77.28
Vested | $ / shares	51.43
Forfeited/Cancelled | $ / shares	0
Nonvested performance-based stock awards/units, Ending balance | $ / shares	$ 59.57
Performance Based Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested performance-based stock awards/units, Beginning balance | shares	13,186
Granted | shares	13,655
Vested | shares	(13,077)
Forfeited/Cancelled | shares	(491)
Nonvested performance-based stock awards/units, Ending balance | shares	13,273
Nonvested performance-based stock awards/units, Beginning balance | $ / shares	$ 51.62
Granted | $ / shares	36.51
Vested | $ / shares	51.50
Forfeited/Cancelled | $ / shares	42.94
Nonvested performance-based stock awards/units, Ending balance | $ / shares	$ 36.51