Income Taxes - Reconciliation From Statutory Rate to Effective Tax (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]
Tax expense at statutory rate of 35%									$ 42,057	$ 36,896	$ 37,065
Increase (decrease) in taxes resulting from:
State tax, net of federal income tax benefit									2,072	1,621	2,268
Tax-exempt interest income									(5,520)	(4,916)	(4,084)
Bank-owned life insurance									(1,871)	(1,972)	(2,309)
Non-qualified stock option expiration									0	0	2,746
Other, net									(297)	156	(320)
Income tax expense (benefit)	$ 9,414	$ 9,034	$ 9,601	$ 8,392	$ 9,968	$ 7,913	$ 6,206	$ 7,698	$ 36,441	$ 31,785	$ 35,366
Statutory interest rate									35.00%	35.00%	35.00%