Income Taxes - Summary of Income Tax Reconciliation (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]
Federal income tax at expected statutory rate									$ 44,413	$ 51,532	$ 62,629
Tax-exempt interest, net of interest incurred to carry tax-exempt securities									(4,864)	(4,406)	(4,876)
Bank owned life insurance									(2,385)	(2,237)	(2,087)
State income tax expense, net of federal tax effect									15,274	12,866	9,904
Tax credits from securities and other investments									(28,988)	(20,555)	(17,408)
Reduction in reserve for uncertainties									0	(6,971)	(1,821)
Other, net									488	833	638
Income tax expense	$ (10,987)	$ 10,179	$ 12,474	$ 12,272	$ 7,827	$ 10,654	$ 11,751	$ 830	$ 23,938	$ 31,062	$ 46,979