Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) (USD $)				9 Months Ended												9 Months Ended						9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member]	Sep. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Minimum [Member] Discount rates [Member]		Sep. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Maximum [Member] Discount rates [Member]		Sep. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Minimum [Member] Probability of default [Member]	Sep. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Minimum [Member] Prepayment rates [Member]	Sep. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Maximum [Member] Probability of default [Member]	Sep. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Maximum [Member] Prepayment rates [Member]	Sep. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Weighted average [Member] Probability of default [Member]	Sep. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Weighted average [Member] Prepayment rates [Member]	Sep. 30, 2013 Equity Securities [Member]	Dec. 31, 2012 Interest Rate Swap [Member]	Sep. 30, 2013 Interest Rate Swap [Member] Option model [Member] Minimum [Member] Counterparty credit risk [Member]		Sep. 30, 2013 Interest Rate Swap [Member] Option model [Member] Maximum [Member] Counterparty credit risk [Member]		Sep. 30, 2013 Impaired Loans [Member]		Sep. 30, 2013 Impaired Loans [Member] Reserve Study [Member] NGL per gallon [Member]		Sep. 30, 2013 Impaired Loans [Member] Reserve Study [Member] Minimum [Member] Gas per MCF [Member]		Sep. 30, 2013 Impaired Loans [Member] Reserve Study [Member] Minimum [Member] Oil per BBL/d [Member]		Sep. 30, 2013 Impaired Loans [Member] Reserve Study [Member] Maximum [Member] Gas per MCF [Member]		Sep. 30, 2013 Impaired Loans [Member] Reserve Study [Member] Maximum [Member] Oil per BBL/d [Member]		Sep. 30, 2013 Impaired Loans [Member] Reserve Study [Member] Weighted average [Member] Discount rates [Member]		Sep. 30, 2013 Other Real Estate Owned [Member]	Sep. 30, 2013 Level 3 [Member]	Dec. 31, 2012 Level 3 [Member]	Sep. 30, 2013 Level 3 [Member] Equity Securities [Member]	Dec. 31, 2012 Level 3 [Member] Equity Securities [Member]	Sep. 30, 2013 Fair Value, Measurements, Nonrecurring [Member]	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member]	Sep. 30, 2013 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member]	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 1,291,063,000	$ 1,171,303,000																																	$ 26,877,000	$ 24,793,000	$ 1,420,000	$ 1,420,000
Other Real Estate Owned Assets Fair Value																																							10,875,000	12,228,000	242,000	247,000
Assets			24,512,000											1,420,000	0					7,727,000	[1]													215,000	26,877,000	24,793,000			73,855,000	102,004,000	14,549,000	7,074,000
Weighted average rate				5.75%	[2]	15.50%	[2]	0.00%	0.00%	100.00%	99.75%	19.61%	8.20%			7.98%	[3]	9.11%	[3]													10.00%	[3]
Fair Value Inputs Monetary																						$ 1.54	[4]	$ 3.56	[4]	$ 79.97	[4]	$ 7.60	[4]	$ 106.00	[4]

[1] 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.

[2] (a)incorporates spread over risk free rate related primarily to credit quality and illiquidity of secu

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

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