Investment Securities (Continuous Unrealized Loss Position, Fair Value) (Detail) $ in Thousands	Jun. 30, 2019 USD ($)
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	$ 16,142
Continuous unrealized losses existing for less than 12 months, unrealized losses	(43)
Continuous unrealized losses existing for greater than 12 months, fair value	947,667
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(18,290)
Total, fair value	963,809
Total, unrealized losses	(18,333)
Held-to-maturity securities
Continuous unrealized losses existing for less than 12 months, fair value	17,922
Continuous unrealized losses existing for less than 12 months, unrealized losses	(104)
Continuous unrealized losses existing for greater than 12 months, fair value	194,609
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(2,814)
Total, fair value	212,531
Total, unrealized losses	(2,918)
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	24,982
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(20)
Total, fair value	24,982
Total, unrealized losses	(20)
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
Held-to-maturity securities
Continuous unrealized losses existing for less than 12 months, fair value	14,221
Continuous unrealized losses existing for less than 12 months, unrealized losses	(68)
Continuous unrealized losses existing for greater than 12 months, fair value	180,104
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(2,535)
Total, fair value	194,325
Total, unrealized losses	(2,603)
Municipal
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	5,512
Continuous unrealized losses existing for less than 12 months, unrealized losses	(3)
Continuous unrealized losses existing for greater than 12 months, fair value	6,566
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(112)
Total, fair value	12,078
Total, unrealized losses	(115)
Held-to-maturity securities
Continuous unrealized losses existing for less than 12 months, fair value	3,701
Continuous unrealized losses existing for less than 12 months, unrealized losses	(36)
Continuous unrealized losses existing for greater than 12 months, fair value	14,505
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(279)
Total, fair value	18,206
Total, unrealized losses	(315)
Corporate notes, financial issuers
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	9,965
Continuous unrealized losses existing for less than 12 months, unrealized losses	(33)
Continuous unrealized losses existing for greater than 12 months, fair value	66,716
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(7,260)
Total, fair value	76,681
Total, unrealized losses	(7,293)
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	665
Continuous unrealized losses existing for less than 12 months, unrealized losses	(7)
Continuous unrealized losses existing for greater than 12 months, fair value	841,068
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(10,720)
Total, fair value	841,733
Total, unrealized losses	(10,727)
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	8,335
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(178)
Total, fair value	8,335
Total, unrealized losses	$ (178)