Investment Securities (Disclosure Regarding Length of Time on Securities Available for Sale with Temporary Impairment) (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value	$ 1,022,127	$ 132,875
Less than 12 months, gross unrealized losses	(11,855)	(919)
12 months or longer, fair value	33	10,007
12 months or longer, gross unrealized losses	0	(52)
Total, fair value	1,022,160	142,882
Total, gross unrealized losses	(11,855)	(971)
U.S. Treasury obligations
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value	49,596
Less than 12 months, gross unrealized losses	(125)
12 months or longer, fair value	0
12 months or longer, gross unrealized losses	0
Total, fair value	49,596
Total, gross unrealized losses	(125)
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value	940,975	127,600
Less than 12 months, gross unrealized losses	(11,447)	(824)
12 months or longer, fair value	33	8,007
12 months or longer, gross unrealized losses	0	(28)
Total, fair value	941,008	135,607
Total, gross unrealized losses	(11,447)	(852)
State and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value	24,846	5,275
Less than 12 months, gross unrealized losses	(147)	(95)
12 months or longer, fair value	0	0
12 months or longer, gross unrealized losses	0	0
Total, fair value	24,846	5,275
Total, gross unrealized losses	(147)	(95)
Corporate obligations
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value	6,710	0
Less than 12 months, gross unrealized losses	(136)	0
12 months or longer, fair value	0	2,000
12 months or longer, gross unrealized losses	0	(24)
Total, fair value	6,710	2,000
Total, gross unrealized losses	$ (136)	$ (24)