Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 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	$ 113,023	$ 229,583
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	455	694
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	449,943	523,463
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	17,617	23,726
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	562,966	753,046
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	18,072	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		2,318
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		2,318
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		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	84,160	111,646
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	435	419
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	300,792	368,706
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	6,065	9,411
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	384,952	480,352
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	6,500	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	27,985	112,473
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	15	229
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	122,833	130,401
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	315	1,079
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	150,818	242,874
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	330	1,308
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	878	3,146
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5	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	878	3,146
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	5	43
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
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	26,318	24,356
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	11,237	13,236
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	26,318	24,356
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 11,237	$ 13,236