Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Mar. 31, 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	$ 787,398	$ 773,932
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(11,298)	(13,175)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	95,465	85,152
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8,962)	(9,326)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	882,863	859,084
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(20,260)	(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	6,014	4,898
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(26)	(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	6,014	4,898
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(26)	(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	33,494	33,883
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(700)	(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	33,494	33,883
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(700)	(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	689,399	670,708
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(10,015)	(11,630)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	66,191	56,200
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2,008)	(2,202)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	755,590	726,908
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(12,023)	(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	14,485	14,534
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(51)	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	14,485	14,534
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(51)	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	16,642	16,632
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(58)	(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	16,642	16,632
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(58)	(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	22,180	33,277
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(443)	(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	22,180	33,277
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(443)	(762)
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	198
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2)
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	198
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2)
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	4,986
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	4,986
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (3)
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percent of Unrealized Losses	34.00%
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	29,274	28,952
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(6,954)	(7,124)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	29,274	28,952
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (6,954)	$ (7,124)