Retirement Plans - Additional Information (Detail) - USD ($)		3 Months Ended		9 Months Ended
	Jul. 01, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Salaries & benefits expense		$ 17,533,000	$ 13,369,000	$ 52,875,000	$ 39,989,000
Employee service period under ESOP				1 year
Contribution and paid out as benefits under ESOP		0	366,000	$ 1,506,000	927,000
Deferred compensation obligations		6,661,000		6,661,000		$ 7,408,000
Earnings credits on deferred balances included in noninterest expense		126,000	124,000	412,000	415,000
Contribution and paid out as benefits		270,000	125,000	945,000	444,000
Salaries And Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expenses related to ESOP		603,000	$ 382,000	$ 1,783,000	$ 1,142,000
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		$ 150,000		$ 150,000
Supplemental Retirement Plans [Member] | December 31, 2015 [Member]
Defined Benefit Plan Disclosure [Line Items]
Contribution and paid out as benefits				$ 1,214,000