Investment Securities (Continuous Unrealized Loss Position, Fair Value) (Detail) $ in Thousands	Sep. 30, 2021 USD ($)
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	$ 929,934
Continuous unrealized losses existing for less than 12 months, unrealized losses	(25,867)
Continuous unrealized losses existing for greater than 12 months, fair value	46,086
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(1,022)
Total, fair value	976,020
Total, unrealized losses	(26,889)
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	54,995
Continuous unrealized losses existing for less than 12 months, unrealized losses	(210)
Continuous unrealized losses existing for greater than 12 months, fair value	111
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(4)
Total, fair value	55,106
Total, unrealized losses	(214)
Corporate notes, financial issuers
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	24,778
Continuous unrealized losses existing for less than 12 months, unrealized losses	(223)
Continuous unrealized losses existing for greater than 12 months, fair value	45,975
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(1,018)
Total, fair value	70,753
Total, unrealized losses	(1,241)
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	850,161
Continuous unrealized losses existing for less than 12 months, unrealized losses	(25,434)
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	850,161
Total, unrealized losses	(25,434)
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