Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) (USD $)	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 US Government Agencies Residential Mortgage-backed Securities [Member]	Dec. 31, 2013 US Government Agencies Residential Mortgage-backed Securities [Member]	Sep. 30, 2014 Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Residential [Member]	Dec. 31, 2013 Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Residential [Member]	Sep. 30, 2014 Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]	Dec. 31, 2013 Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]	Sep. 30, 2014 Obligations of States and Political Subdivisions [Member]	Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member]	Dec. 31, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member]	Sep. 30, 2014 Equity Securities [Member]	Sep. 30, 2013 Equity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months	$ 446,488,000	$ 820,209,000	$ 2,862,000	$ 2,035,000	$ 282,378,000	$ 632,231,000	$ 159,666,000	$ 183,542,000	$ 1,582,000	$ 0	$ 2,401,000
Grous Unrealized Losses Less Than 12 Months	(1,758,000)	(24,588,000)	(10,000)	(59,000)	(1,311,000)	(22,844,000)	(434,000)	(1,448,000)	(3,000)	0	(237,000)
Estimated Fair Value, 12 Months or More	527,989,000	110,947,000	0	0	397,581,000	65,324,000	105,451,000	24,501,000	0	24,957,000	21,122,000
Gross Unrealized Losses 12 Months or More	(28,892,000)	(23,078,000)	0	0	(15,103,000)	(4,319,000)	(1,026,000)	(479,000)	0	(12,763,000)	(18,280,000)
Total Estimated Fair Value	974,477,000	931,156,000	2,862,000	2,035,000	679,959,000	697,555,000	265,117,000	208,043,000	1,582,000	24,957,000	23,523,000	0	0
Total Gross Unrealized Losses	$ (30,650,000)	$ (47,666,000)	$ (10,000)	$ (59,000)	$ (16,414,000)	$ (27,163,000)	$ (1,460,000)	$ (1,927,000)	$ (3,000)	$ (12,763,000)	$ (18,517,000)