Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 668,962	$ 431,247
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(13,517)	(3,386)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	420,472	456,539
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(18,208)	(15,148)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	1,089,434	887,786
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(31,725)	(18,534)
US Government Agencies Residential Mortgage-backed Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	7,923	5,584
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(190)	(21)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	7,923	5,584
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(190)	(21)
US Government Agency Debt Securities Commercial [Domain]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	95,862	48,322
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,120)	(962)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	31,813	32,683
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,356)	(894)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	127,675	81,005
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3,476)	(1,856)
Residential Mortgage Backed Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	500,625	351,222
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(10,277)	(2,295)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	378,709	400,984
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(15,836)	(9,746)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	879,334	752,206
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(26,113)	(12,041)
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	13,560	13,985
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(299)	(71)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	13,560	13,985
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(299)	(71)
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	999	997
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	(1)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	99	99
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1)	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	1,098	1,096
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2)	(1)
Obligations of States and Political Subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	31,032	7,144
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(413)	(32)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	3,489	3,653
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(293)	(129)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	34,521	10,797
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(706)	(161)
Foreign Government Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	197
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	197
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3)
Corporate Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	18,764	3,993
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(214)	(4)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	18,764	3,993
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (214)	(4)
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percent of Unrealized Losses	2.00%
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 0	0
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	6,362	19,120
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(722)	(4,379)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	6,362	19,120
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (722)	$ (4,379)