Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Jun. 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	$ 706,468	$ 773,932
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(8,671)	(13,175)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	92,573	85,152
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8,997)	(9,326)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	799,041	859,084
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(17,668)	(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	5,825	4,898
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(20)	(11)
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	5,825	4,898
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(20)	(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	58,053	33,883
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(727)	(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	58,053	33,883
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(727)	(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	634,704	670,708
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(7,786)	(11,630)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	63,386	56,200
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,804)	(2,202)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	698,090	726,908
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(9,590)	(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		14,534
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		142
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		14,534
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		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	100	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	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	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	7,786	33,277
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(138)	(762)
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	7,786	33,277
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(138)	(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	41.00%
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	29,187	28,952
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(7,193)	(7,124)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	29,187	28,952
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (7,193)	$ (7,124)