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		9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Minimum [Member] Discount rates [Member]		Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Maximum [Member] Discount rates [Member]		Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Minimum [Member] Probability of default [Member]	Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Minimum [Member] Prepayment rates [Member]	Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Maximum [Member] Probability of default [Member]	Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Maximum [Member] Prepayment rates [Member]	Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Weighted average [Member] Probability of default [Member]	Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Discounted Cash Flow [Member] Weighted average [Member] Prepayment rates [Member]	Dec. 31, 2013 Interest Rate Swap [Member]	Sep. 30, 2014 Interest Rate Swap [Member] Option model [Member] Minimum [Member] Counterparty credit risk [Member]		Sep. 30, 2014 Interest Rate Swap [Member] Option model [Member] Maximum [Member] Counterparty credit risk [Member]		Sep. 30, 2014 Impaired Loans [Member] Reserve Study [Member] NGL per gallon [Member]		Sep. 30, 2014 Impaired Loans [Member] Reserve Study [Member] Minimum [Member] Gas per MCF [Member]		Sep. 30, 2014 Impaired Loans [Member] Reserve Study [Member] Minimum [Member] Oil per BBL/d [Member]		Sep. 30, 2014 Impaired Loans [Member] Reserve Study [Member] Maximum [Member] Gas per MCF [Member]		Sep. 30, 2014 Impaired Loans [Member] Reserve Study [Member] Maximum [Member] Oil per BBL/d [Member]		Sep. 30, 2014 impaired Loans Valued Using a Reserve Study [Domain]	Sep. 30, 2014 impaired Loans Valued Using a Reserve Study [Domain] Reserve Study [Member] Weighted average [Member] Discount rates [Member]	Sep. 30, 2014 impaired Loans Valued Using Discounted Cash Flows [Member]	Sep. 30, 2014 Impaired Loans Valued Using Discounted Cash Flows [Domain] Reserve Study [Member] Weighted average [Member] Discount rates [Member]	Sep. 30, 2014 Level 3 [Member]	Dec. 31, 2013 Level 3 [Member]	Sep. 30, 2014 Level 3 [Member] Equity Securities [Member]	Dec. 31, 2013 Level 3 [Member] Equity Securities [Member]	Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member]	Dec. 31, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member]	Dec. 31, 2013 Pooled Trust Preferred Collateralized Debt Obligations [Member] Level 3 [Member]	Sep. 30, 2014 Pooled Trust Preferred Collateralized Debt Obligations [Member] Level 3 [Member]	Sep. 30, 2014 Fair Value, Measurements, Nonrecurring [Member]	Dec. 31, 2013 Fair Value, Measurements, Nonrecurring [Member]	Sep. 30, 2014 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member]	Dec. 31, 2013 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member]	Sep. 30, 2014 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member] Other Real Estate Owned [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value	$ 1,332,774,000	$ 1,318,365,000																														$ 30,876,000	$ 24,943,000	$ 1,420,000	$ 1,420,000	$ 29,456,000	$ 23,523,000	$ 23,523,000	$ 29,456,000
Other Real Estate Owned Assets Fair Value																																								8,309,000	12,924,000	190,000	172,000	190,000
Assets													0															5,934,000		113,000		30,876,000	24,943,000							48,935,000	63,483,000	13,779,000	13,828,000
Weighted average rate			5.50%	[1]	14.50%	[1]	0.00%	0.00%	100.00%	100.00%	17.48%	6.35%		3.74%	[2]	6.62%	[2]												10.00%		9.00%
Fair Value Inputs Monetary																		$ 0.83	[3]	$ 3.84	[3]	$ 83.64	[3]	$ 6.45	[3]	$ 107.00	[3]

[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.