Income Taxes (Income Tax Computed by Applying U.S. Federal Statutory Income Tax Rate) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income tax expense computed at the statutory rate	$ 73,777	$ 104,888	$ 106,823
Effect of tax-exempt interest	(51,624)	(49,166)	(49,700)
Tax benefit on dividends paid in our 401k plan	(1,851)	(1,743)	(1,551)
Bank owned life insurance income	(783)	(774)	(710)
Non-deductible compensation	1,123	1,708	210
Non-deductible FDIC premiums	1,790	1,267	1,771
Non-deductible meals and entertainment	786	1,299	1,193
Net tax benefit from stock-based compensation	(852)	(2,447)	(3,865)
Asset contribution to a charitable trust	(2,556)	0	0
Deferred tax adjustment related to reduction in U.S. federal statutory income tax rate	0	0	(231)
Other	360	838	(177)
Income tax expense, as reported	$ 20,170	$ 55,870	$ 53,763