Income Taxes (Income Tax Computed by Applying U.S. Federal Statutory Income Tax Rate) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax expense computed at the statutory rate	$ 142,927	$ 119,494	$ 111,930
Effect of tax-exempt interest	(81,995)	(75,696)	(70,889)
Bank owned life insurance income	(1,116)	(1,260)	(1,255)
Net tax benefit from stock-based compensation	(9,062)	(5,063)	0
Provisional deferred tax adjustment related to reduction in U.S. federal statutory income tax rate	(4,047)	0	0
Correction for prior year tax-exempt interest	(2,906)	0	0
Other	413	(325)	685
Income tax expense, as reported	$ 44,214	$ 37,150	$ 40,471