Investment Securities (Continuous Unrealized Loss Position, Fair Value) (Detail) $ in Thousands	Sep. 30, 2018 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	$ 841,018
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(22,007)
Continuous unrealized losses existing for greater than 12 months, Fair value	1,220,743
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(75,459)
Total, Fair value	2,061,761
Total, Unrealized losses	(97,466)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	445,486
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(16,171)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	450,855
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	(39,299)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	896,341
Held-to-maturity Securities, Continuous Unrealized Loss Position, Unrealized Losses	(55,470)
U.S. Treasury
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	100,008
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(131)
Continuous unrealized losses existing for greater than 12 months, Fair value	24,753
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(261)
Total, Fair value	124,761
Total, Unrealized losses	(392)
U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	37,672
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(914)
Continuous unrealized losses existing for greater than 12 months, Fair value	70,032
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(71)
Total, Fair value	107,704
Total, Unrealized losses	(985)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	298,736
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(12,317)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	366,981
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	(35,389)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	665,717
Held-to-maturity Securities, Continuous Unrealized Loss Position, Unrealized Losses	(47,706)
Municipal Securities
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	38,774
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(553)
Continuous unrealized losses existing for greater than 12 months, Fair value	20,489
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(561)
Total, Fair value	59,263
Total, Unrealized losses	(1,114)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	146,750
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(3,854)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	83,874
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	(3,910)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	230,624
Held-to-maturity Securities, Continuous Unrealized Loss Position, Unrealized Losses	(7,764)
Corporate notes, Financial issuers
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	68,639
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(4,475)
Continuous unrealized losses existing for greater than 12 months, Fair value	3,730
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(244)
Total, Fair value	72,369
Total, Unrealized losses	(4,719)
Corporate notes, Other
Schedule of Available-for-sale Securities [Line Items]
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
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	573,401
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(14,706)
Continuous unrealized losses existing for greater than 12 months, Fair value	1,079,068
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(73,244)
Total, Fair value	1,652,469
Total, Unrealized losses	(87,950)
Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	22,524
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(1,228)
Continuous unrealized losses existing for greater than 12 months, Fair value	22,671
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(1,078)
Total, Fair value	45,195
Total, Unrealized losses	$ (2,306)