Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 61,472	$ 493,938
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(162)	(3,881)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	90,805	293,076
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8,507)	(14,910)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	152,277	787,014
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(8,669)	(18,791)
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		6,798
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(20)
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		6,798
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(20)
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		16,596
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(247)
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		16,596
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(247)
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	59,691	436,011
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(137)	(3,293)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	60,909	263,119
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(419)	(7,413)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	120,600	699,130
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(556)	(10,706)
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		15,009
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		178
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		15,009
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		178
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		12,316
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(85)
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		12,316
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(85)
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	1,781	7,208
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(25)	(58)
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	1,781	7,208
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (25)	(58)
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percent of Unrealized Losses	93.00%
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	29,896	29,957
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8,088)	(7,497)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	29,896	29,957
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (8,088)	$ (7,497)