Investment Securities (Continuous Unrealized Loss Position, Fair Value) (Detail) $ in Thousands	Jun. 30, 2018 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	$ 771,212
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(21,244)
Continuous unrealized losses existing for greater than 12 months, Fair value	1,053,439
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(57,607)
Total, Fair value	1,824,651
Total, Unrealized losses	(78,851)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	474,384
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(14,871)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	341,210
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	(25,048)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	815,594
Held-to-maturity Securities, Continuous Unrealized Loss Position, Unrealized Losses	(39,919)
U.S. Treasury
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	30,216
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(290)
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	30,216
Total, Unrealized losses	(290)
U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	11,127
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(87)
Continuous unrealized losses existing for greater than 12 months, Fair value	99,799
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(214)
Total, Fair value	110,926
Total, Unrealized losses	(301)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	317,261
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(11,169)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	288,132
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	(22,871)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	605,393
Held-to-maturity Securities, Continuous Unrealized Loss Position, Unrealized Losses	(34,040)
Municipal Securities
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	31,744
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(438)
Continuous unrealized losses existing for greater than 12 months, Fair value	14,660
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(402)
Total, Fair value	46,404
Total, Unrealized losses	(840)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	157,123
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	(3,702)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	53,078
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	(2,177)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	210,201
Held-to-maturity Securities, Continuous Unrealized Loss Position, Unrealized Losses	(5,879)
Corporate notes, Financial issuers
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	70,557
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(3,759)
Continuous unrealized losses existing for greater than 12 months, Fair value	3,720
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(253)
Total, Fair value	74,277
Total, Unrealized losses	(4,012)
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	600,141
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(15,423)
Continuous unrealized losses existing for greater than 12 months, Fair value	914,167
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(55,392)
Total, Fair value	1,514,308
Total, Unrealized losses	(70,815)
Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	27,427
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(1,247)
Continuous unrealized losses existing for greater than 12 months, Fair value	21,093
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(1,346)
Total, Fair value	48,520
Total, Unrealized losses	$ (2,593)