Investment Securities (Continuous Unrealized Loss Position, Fair Value) (Detail) $ in Thousands	Mar. 31, 2021 USD ($)
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	$ 610,997
Continuous unrealized losses existing for less than 12 months, unrealized losses	(26,355)
Continuous unrealized losses existing for greater than 12 months, fair value	48,367
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(790)
Total, fair value	659,364
Total, unrealized losses	(27,145)
U.S. Treasury
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	0
Continuous unrealized losses existing for less than 12 months, unrealized losses	0
Continuous unrealized losses existing for greater than 12 months, fair value	0
Continuous unrealized losses existing for greater than 12 months, unrealized losses	0
Total, fair value	0
Total, unrealized losses	0
U.S. Government agencies
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	0
Continuous unrealized losses existing for less than 12 months, unrealized losses	0
Continuous unrealized losses existing for greater than 12 months, fair value	0
Continuous unrealized losses existing for greater than 12 months, unrealized losses	0
Total, fair value	0
Total, unrealized losses	0
Municipal
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	24,326
Continuous unrealized losses existing for less than 12 months, unrealized losses	(168)
Continuous unrealized losses existing for greater than 12 months, fair value	110
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(4)
Total, fair value	24,436
Total, unrealized losses	(172)
Corporate notes, financial issuers
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	14,850
Continuous unrealized losses existing for less than 12 months, unrealized losses	(150)
Continuous unrealized losses existing for greater than 12 months, fair value	48,196
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(785)
Total, fair value	63,046
Total, unrealized losses	(935)
Corporate notes, other
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	0
Continuous unrealized losses existing for less than 12 months, unrealized losses	0
Continuous unrealized losses existing for greater than 12 months, fair value	0
Continuous unrealized losses existing for greater than 12 months, unrealized losses	0
Total, fair value	0
Total, unrealized losses	0
Mortgage-backed securities
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	571,821
Continuous unrealized losses existing for less than 12 months, unrealized losses	(26,037)
Continuous unrealized losses existing for greater than 12 months, fair value	61
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(1)
Total, fair value	571,882
Total, unrealized losses	(26,038)
Collateralized mortgage obligations
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	0
Continuous unrealized losses existing for less than 12 months, unrealized losses	0
Continuous unrealized losses existing for greater than 12 months, fair value	0
Continuous unrealized losses existing for greater than 12 months, unrealized losses	0
Total, fair value	0
Total, unrealized losses	$ 0