Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) (USD $)				6 Months Ended												6 Months Ended						6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member]	Jun. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Minimum [Member] Discount rates [Member]		Jun. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Maximum [Member] Discount rates [Member]		Jun. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Minimum [Member] Probability of default [Member]	Jun. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Minimum [Member] Prepayment rates [Member]	Jun. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Maximum [Member] Probability of default [Member]	Jun. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Maximum [Member] Prepayment rates [Member]	Jun. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Weighted average [Member] Probability of default [Member]	Jun. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Weighted average [Member] Prepayment rates [Member]	Jun. 30, 2013 Equity Securities [Member]	Dec. 31, 2012 Interest Rate Swap [Member]	Jun. 30, 2013 Interest Rate Swap [Member] Option model [Member] Minimum [Member] Counterparty credit risk [Member]		Jun. 30, 2013 Interest Rate Swap [Member] Option model [Member] Maximum [Member] Counterparty credit risk [Member]		Jun. 30, 2013 Impaired Loans [Member]		Jun. 30, 2013 Impaired Loans [Member] Reserve Study [Member] NGL per gallon [Member]		Jun. 30, 2013 Impaired Loans [Member] Reserve Study [Member] Minimum [Member] Gas per MCF [Member]		Jun. 30, 2013 Impaired Loans [Member] Reserve Study [Member] Minimum [Member] Oil per BBL/d [Member]		Jun. 30, 2013 Impaired Loans [Member] Reserve Study [Member] Maximum [Member] Gas per MCF [Member]		Jun. 30, 2013 Impaired Loans [Member] Reserve Study [Member] Maximum [Member] Oil per BBL/d [Member]		Jun. 30, 2013 Impaired Loans [Member] Reserve Study [Member] Weighted average [Member] Discount rates [Member]		Jun. 30, 2013 Other Real Estate Owned [Member]	Jun. 30, 2013 Level 3 [Member]	Dec. 31, 2012 Level 3 [Member]	Jun. 30, 2013 Level 3 [Member] Equity Securities [Member]	Dec. 31, 2012 Level 3 [Member] Equity Securities [Member]	Jun. 30, 2013 Fair Value, Measurements, Nonrecurring [Member]	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member]	Jun. 30, 2013 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member]	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member]	Jun. 30, 2013 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member] Other Real Estate Owned [Member]	Jun. 30, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Level 3 [Member]	Dec. 31, 2012 Pooled Trust Preferred Collateralized Debt Obligations [Member] Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value of Available for Sale	$ 1,306,687,000	$ 1,171,303,000																																	$ 27,289,000	$ 24,793,000	$ 1,420,000	$ 1,420,000						$ 25,869,000	$ 23,373,000
Other Real Estate Owned Assets Fair Value																																							17,303,000	12,228,000		247,000	187,000
Assets		1,216,011,000	24,512,000											1,420,000	0					715,000	[1]													215,000	27,289,000	24,793,000			80,746,000	102,004,000	7,727,000	7,074,000
Weighted average rate				6.00%	[2]	16.00%	[2]	0.00%	0.00%	100.00%	100.00%	19.32%	7.48%			6.80%	[3]	9.12%	[3]													10.00%	[3]
Fair Value Inputs Monetary																						$ 1.54	[4]	$ 3.67	[4]	$ 90.97	[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.