Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 97,754	$ 229,583
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	306	694
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	398,971	523,463
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	11,929	23,726
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	496,725	753,046
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	12,235	24,420
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	2,240	2,318
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	3
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	2,240	2,318
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	0	3
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	78,300	111,646
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	242	419
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	277,333	368,706
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4,610	9,411
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	355,633	480,352
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	4,852	9,830
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	10,091	112,473
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9	229
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	91,570	130,401
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	82	1,079
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	101,661	242,874
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 91	1,308
Obligations of States and Political Subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percent of Unrealized Losses	1.00%
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 7,123	3,146
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	55	43
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	7,123	3,146
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 55	43
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percent of Unrealized Losses	59.00%
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	30,068	24,356
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	7,237	13,236
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	30,068	24,356
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 7,237	$ 13,236