Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 478,939	$ 773,932
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3,650)	(13,175)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	259,307	85,152
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(11,601)	(9,326)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	738,246	859,084
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(15,251)	(22,501)
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		4,898
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(11)
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		4,898
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(11)
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	82,134	33,883
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,078)	(561)
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	82,134	33,883
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1,078)	(561)
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	377,725	670,708
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,526)	(11,630)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	226,562	56,200
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(5,033)	(2,202)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	604,287	726,908
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(7,559)	(13,832)
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	14,222	14,534
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(4)	142
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	14,222	14,534
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(4)	142
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	0	16,632
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(69)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	100	0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	100	16,632
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	0	(69)
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	4,858	33,277
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(42)	(762)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2,591	0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(87)	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	7,449	33,277
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(129)	(762)
Corporate Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 0
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percent of Unrealized Losses	42.00%
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	30,054	28,952
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(6,481)	(7,124)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	30,054	28,952
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (6,481)	$ (7,124)