Retirement Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Salaries & benefits expense	$ 82,930,000	$ 80,724,000	$ 71,405,000
Employee service period under ESOP	1 year
Contribution and paid out as benefits under ESOP	$ 2,073,000	1,368,000	2,651,000
Deferred compensation obligations	6,605,000	6,525,000
Earnings credits on deferred balances included in noninterest expense	478,000	487,000	538,000
Cash values of the insurance policies purchased to fund deferred compensation obligations	97,783,000	95,912,000	94,560,000
Unfunded status of the supplemental retirement plans	(5,352,000)	(4,714,000)
Net actuarial loss	509,000
Salaries and Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expenses related to ESOP	$ 2,149,000	1,831,000	2,282,000
401(k) Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Minimum age for employees	21 years
Minimum period for employees to participate	90 days
Discretionary matching contribution equal to percentage of participant's contribution	50.00%
Discretionary matching contribution maximum percentage of eligible compensation	4.00%
Salaries & benefits expense	$ 776,000	678,000	300,000
Employer's contribution to retirement plan	767,000	811,000	$ 0
Supplemental Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash values of the insurance policies purchased to fund deferred compensation obligations	97,783,000	95,912,000
Unfunded status of the supplemental retirement plans	5,352,000	4,714,000
Accumulated other comprehensive loss	3,102,000	2,732,000
Deferred tax asset	$ 2,250,000	$ 1,982,000