Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) (USD $)			12 Months Ended																																				12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Probability of default [Member] Minimum [Member]	Dec. 31, 2013 Probability of default [Member] Maximum [Member]	Dec. 31, 2013 Probability of default [Member] Weighted Average [Member]	Dec. 31, 2013 Prepayment rates [Member]	Dec. 31, 2013 Prepayment rates [Member] Minimum [Member]	Dec. 31, 2013 Prepayment rates [Member] Maximum [Member]	Dec. 31, 2013 Prepayment rates [Member] Weighted Average [Member]	Dec. 31, 2013 Discount rates [Member]	Dec. 31, 2013 Gas per MCF [Member]	Dec. 31, 2013 Gas per MCF [Member] Minimum [Member]		Dec. 31, 2013 Gas per MCF [Member] Maximum [Member]		Dec. 31, 2013 Oil per BBL/d [Member]	Dec. 31, 2013 Oil per BBL/d [Member] Minimum [Member]		Dec. 31, 2013 Oil per BBL/d [Member] Maximum [Member]		Dec. 31, 2013 NGL per gallon [Member]	Dec. 31, 2013 NGL per gallon [Member] Weighted Average [Member]	Dec. 31, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Probability of default [Member]	Dec. 31, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discount rates [Member] Minimum [Member]		Dec. 31, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discount rates [Member] Maximum [Member]		Dec. 31, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member]	Dec. 31, 2013 Other Investments [Member]	Dec. 31, 2013 Other Investments [Member] Par Value [Member]	Dec. 31, 2013 Interest rate swap [Member] Minimum [Member]		Dec. 31, 2013 Interest rate swap [Member] Maximum [Member]		Dec. 31, 2013 Interest rate swap [Member] Counterparty credit risk [Member]	Dec. 31, 2013 Interest rate swap [Member] Option model [Member]	Dec. 31, 2013 Impaired Loans [Member]		Dec. 31, 2013 Impaired Loans [Member] Weighted Average [Member]	Dec. 31, 2013 Impaired Loans [Member] Discount rates [Member]	Dec. 31, 2013 Impaired Loans [Member] Reserve study [Member]	Dec. 31, 2013 Other Real Estate Owned [Member] Counterparty credit risk [Member]	Dec. 31, 2013 Other Real Estate Owned [Member] Internal Valuation [Member]	Dec. 31, 2013 Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2013 Fair Value, Inputs, Level 3 [Member] Interest rate swap [Member]	Dec. 31, 2013 Equities [Member]	Dec. 31, 2012 Equities [Member]	Dec. 31, 2012 Equities [Member] Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2013 Equities [Member] Fair Value, Inputs, Level 3 [Member] Other Investments [Member]	Dec. 31, 2013 Fair Value, Measurements, Nonrecurring [Member]	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member]	Dec. 31, 2013 Fair Value, Measurements, Nonrecurring [Member] Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member] Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned																																																			$ 12,924,000	$ 12,228,000	$ 172,000	$ 247,000
Other Assets	14,358,000	16,480,000																																											0	0
Available-for-sale Securities, Fair Value Disclosure	1,318,365,000	1,171,303,000																																										24,943,000	24,793,000		1,420,000	1,975,000	1,420,000	1,420,000
Total Debt Securities	1,316,945,000	1,169,328,000																																										23,523,000	23,373,000
Weighted average rate			0.00%	100.00%	19.08%		0.00%	74.35%	7.50%															5.75%	[1]	15.50%	[1]				7.89%	[2]	8.92%	[2]					10.00%
Weighted average												3.56	[3]	7.60	[3]		79.27	[3]	106.00	[3]		1.54
Unobservable Inputs						Prepayment rates				Discount rates	Gas per MCF					Oil per BBL/d					NGL per gallon		Probability of default												Counterparty credit risk					Discount rate
Valuation Technique																												Discounted Cash Flow		Par Value						Option model					Gas Reserve study		Internal Valuation
Fair Value	$ 1,368,167,000	$ 1,216,011,000																																			$ 6,868,000	[4]						$ 24,943,000	$ 24,793,000						$ 63,483,000	$ 102,004,000	$ 13,828,000	$ 7,074,000

[1] incorporates spread over risk free rate related primarily to credit quality and illiquidity of securities.

[2] represents the range of the credit spread curve used in valuation.

[3] unobservable inputs are defined as follows: MCF—million cubic feet; BBL/d—barrels per day; NGL—natural gas liquid.

[4] the remainder of impaired loans valued using Level 3 inputs are not included in this disclosure as the values of those loans are based on bankruptcy agreement documentation.