Investment Securities (Continuous Unrealized Loss Position, Fair Value) (Detail) $ in Thousands	Sep. 30, 2020 USD ($)
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	$ 149,403
Continuous unrealized losses existing for less than 12 months, unrealized losses	(143)
Continuous unrealized losses existing for greater than 12 months, fair value	70,392
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(4,114)
Total, fair value	219,795
Total, unrealized losses	(4,257)
U.S. Treasury
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	109,968
Continuous unrealized losses existing for less than 12 months, unrealized losses	(1)
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	109,968
Total, unrealized losses	(1)
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	38,446
Continuous unrealized losses existing for less than 12 months, unrealized losses	(132)
Continuous unrealized losses existing for greater than 12 months, fair value	113
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(2)
Total, fair value	38,559
Total, unrealized losses	(134)
Corporate notes, financial issuers
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	69,869
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(4,110)
Total, fair value	69,869
Total, unrealized losses	(4,110)
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	989
Continuous unrealized losses existing for less than 12 months, unrealized losses	(10)
Continuous unrealized losses existing for greater than 12 months, fair value	62
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(1)
Total, fair value	1,051
Total, unrealized losses	(11)
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	348
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(1)
Total, fair value	348
Total, unrealized losses	$ (1)