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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Statutory federal income tax expense									$ 55,750	$ 58,122	$ 64,539
Tax-exempt interest income									(15,405)	(13,861)	(14,146)
State and local income taxes, net of federal tax benefits									1,599	1,403	1,887
Federal tax credits									(688)	(623)	(1,704)
Other									1,956	367	(143)
Total tax expense	$ 10,330	$ 8,763	$ 9,732	$ 14,387	$ 9,825	$ 12,720	$ 14,298	$ 8,565	$ 43,212	$ 45,408	$ 50,433