Investment Securities (Investment Securities, Continuous Unrealized Loss Position, Fair Value) (Detail) $ in Thousands	Mar. 31, 2018 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	$ 682,752
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(16,036)
Continuous unrealized losses existing for greater than 12 months, Fair value	1,113,035
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(53,554)
Total, Fair value	1,795,787
Total, Unrealized losses	(69,590)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	471,177
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(11,251)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	341,929
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	(20,098)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	813,106
Held-to-maturity Securities, Continuous Unrealized Loss Position, Unrealized Losses	(31,349)
U.S. Treasury
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	24,727
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(295)
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	24,727
Total, Unrealized losses	(295)
U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	14,742
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(104)
Continuous unrealized losses existing for greater than 12 months, Fair value	134,594
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(459)
Total, Fair value	149,336
Total, Unrealized losses	(563)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	311,150
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(7,688)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	292,800
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	(18,203)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	603,950
Held-to-maturity Securities, Continuous Unrealized Loss Position, Unrealized Losses	(25,891)
Municipal Securities
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	35,225
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(494)
Continuous unrealized losses existing for greater than 12 months, Fair value	13,794
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(362)
Total, Fair value	49,019
Total, Unrealized losses	(856)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	160,027
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(3,563)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	49,129
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	(1,895)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	209,156
Held-to-maturity Securities, Continuous Unrealized Loss Position, Unrealized Losses	(5,458)
Corporate notes, Financial issuers
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	73,037
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(1,294)
Continuous unrealized losses existing for greater than 12 months, Fair value	5,665
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(301)
Total, Fair value	78,702
Total, Unrealized losses	(1,595)
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	999
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(1)
Total, Fair value	999
Total, Unrealized losses	(1)
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	495,082
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(12,177)
Continuous unrealized losses existing for greater than 12 months, Fair value	945,641
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(51,900)
Total, Fair value	1,440,723
Total, Unrealized losses	(64,077)
Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	39,939
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(1,672)
Continuous unrealized losses existing for greater than 12 months, Fair value	12,342
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(531)
Total, Fair value	52,281
Total, Unrealized losses	$ (2,203)