Reconciliation Between Income Tax Expense and Amount Computed by Applying Federal Statutory Tax Rate (Detail) - USD ($) $ in Thousands		3 Months Ended										12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	[2]	Mar. 31, 2016	[2]	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Statutory federal income tax expense												$ 72,451	$ 55,750	$ 58,122
Tax-exempt interest income												(20,196)	(15,405)	(13,861)
Tax-exempt life insurance related income												(3,405)	(932)	(443)
State and local income taxes, net of federal tax benefits												1,462	1,599	1,403
Federal tax credits, net of amortization of LIHTC benefits	[1]											(2,480)	(688)	(623)
Other												371	2,888	810
Total tax expense		$ 11,028	$ 11,984	$ 12,796		$ 12,395		$ 10,330	$ 8,763	$ 9,732	$ 14,387	$ 48,203	$ 43,212	$ 45,408

[1] Low income housing tax credits

[2] During the third quarter of 2016, the Company early adopted ASU No. 2016-09 with an effective date of January 1, 2016. As part of the adoption of this standard, the Company made an accounting policy election to account for stock compensation forfeitures on an actual basis and discontinue the use of an estimated forfeiture approach. This change required a modified retrospective adoption, via a cumulative effect adjustment and recasting of first quarter and second quarter 2016 operating results. The impact of this adoption was an increase to net income of $158 thousand and $220 thousand for the first and second quarters, respectively. Additionally, basic and diluted net income per share increased $0.01 for both periods.