Investment Securities (Investment Securities, Continuous Unrealized Loss Position, Fair Value) (Detail) $ in Thousands	Jun. 30, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	$ 1,372,985
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(33,652)
Continuous unrealized losses existing for greater than 12 months, Fair value	53,497
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(1,478)
Total, Fair value	1,426,482
Total, Unrealized losses	(35,130)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	437,139
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(8,741)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	437,139
Held-to-maturity Securities, Continuous Unrealized Loss Position, Unrealized Losses	(8,741)
U.S. Treasury
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	119,081
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(723)
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	119,081
Total, Unrealized losses	(723)
U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	152,149
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(674)
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	152,149
Total, Unrealized losses	(674)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	363,692
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(7,461)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	363,692
Held-to-maturity Securities, Continuous Unrealized Loss Position, Unrealized Losses	(7,461)
Municipal Securities
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	145,960
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(155)
Continuous unrealized losses existing for greater than 12 months, Fair value	5,852
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(109)
Total, Fair value	151,812
Total, Unrealized losses	(264)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	73,447
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(1,280)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	73,447
Held-to-maturity Securities, Continuous Unrealized Loss Position, Unrealized Losses	(1,280)
Corporate notes, Financial issuers
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	35,154
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(810)
Total, Fair value	35,154
Total, Unrealized losses	(810)
Corporate notes, Other
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	997
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(3)
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	997
Total, Unrealized losses	(3)
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	932,800
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(31,704)
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	932,800
Total, Unrealized losses	(31,704)
Mortgage-backed, Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	11,809
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(122)
Continuous unrealized losses existing for greater than 12 months, Fair value	7,353
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(197)
Total, Fair value	19,162
Total, Unrealized losses	(319)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	10,189
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(271)
Continuous unrealized losses existing for greater than 12 months, Fair value	5,138
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(362)
Total, Fair value	15,327
Total, Unrealized losses	$ (633)