Income Taxes (Income Tax Computed by Applying U.S. Federal Statutory Income Tax Rate) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income tax expense computed at the statutory rate	$ 106,823	$ 142,927	$ 119,494
Effect of tax-exempt interest	(49,700)	(81,034)	(75,369)
Tax benefit on dividends paid in our 401k plan	(1,551)	(2,372)	(2,558)
Bank owned life insurance income	(710)	(1,116)	(1,260)
Non-deductible FDIC premiums	1,771	0	0
Non-deductible meals and entertainment	1,193	983	1,065
Net tax benefit from stock-based compensation	(3,865)	(9,062)	(5,063)
Deferred tax adjustment related to reduction in U.S. federal statutory income tax rate	(231)	(4,047)	0
Correction for prior year tax-exempt interest	0	(2,906)	0
Other	33	841	841
Income tax expense, as reported	$ 53,763	$ 44,214	$ 37,150