Investment Securities (Continuous Unrealized Loss Position, Fair Value) (Detail) $ in Thousands	Mar. 31, 2019 USD ($)
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	$ 16,435
Continuous unrealized losses existing for less than 12 months, unrealized losses	(23)
Continuous unrealized losses existing for greater than 12 months, fair value	1,441,224
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(33,156)
Total, fair value	1,457,659
Total, unrealized losses	(33,179)
Held-to-maturity securities
Continuous unrealized losses existing for less than 12 months, fair value	7,951
Continuous unrealized losses existing for less than 12 months, unrealized losses	(111)
Continuous unrealized losses existing for greater than 12 months, fair value	453,392
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(15,350)
Total, fair value	461,343
Total, unrealized losses	(15,461)
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	24,908
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(97)
Total, fair value	24,908
Total, unrealized losses	(97)
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	208
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(2)
Total, fair value	208
Total, unrealized losses	(2)
Held-to-maturity 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	403,196
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(14,580)
Total, fair value	403,196
Total, unrealized losses	(14,580)
Municipal
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	6,448
Continuous unrealized losses existing for less than 12 months, unrealized losses	(12)
Continuous unrealized losses existing for greater than 12 months, fair value	15,087
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(206)
Total, fair value	21,535
Total, unrealized losses	(218)
Held-to-maturity securities
Continuous unrealized losses existing for less than 12 months, fair value	7,951
Continuous unrealized losses existing for less than 12 months, unrealized losses	(111)
Continuous unrealized losses existing for greater than 12 months, fair value	50,196
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(770)
Total, fair value	58,147
Total, unrealized losses	(881)
Corporate notes, financial issuers
Available-for-sale securities
Continuous unrealized losses existing for less than 12 months, fair value	9,987
Continuous unrealized losses existing for less than 12 months, unrealized losses	(11)
Continuous unrealized losses existing for greater than 12 months, fair value	72,283
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(4,791)
Total, fair value	82,270
Total, unrealized losses	(4,802)
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	0
Continuous unrealized losses existing for less than 12 months, unrealized losses	0
Continuous unrealized losses existing for greater than 12 months, fair value	1,315,030
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(27,662)
Total, fair value	1,315,030
Total, unrealized losses	(27,662)
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	13,708
Continuous unrealized losses existing for greater than 12 months, unrealized losses	(398)
Total, fair value	13,708
Total, unrealized losses	$ (398)