Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Jun. 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	$ 255,185	$ 229,583
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,475	694
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	436,416	523,463
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	15,275	23,726
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	691,601	753,046
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	17,750	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	2,243	2,318
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	24	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	2,243	2,318
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	24	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	225,033	111,646
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,057	419
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	287,128	368,706
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	7,775	9,411
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	512,161	480,352
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	9,832	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	6,793	112,473
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	7	229
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	119,429	130,401
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	221	1,079
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	126,222	242,874
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	228	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	21,116	3,146
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	387	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	21,116	3,146
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	387	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	29,859	24,356
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	7,279	13,236
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	29,859	24,356
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 7,279	$ 13,236