Long -Term Borrowings - Debentures and Related Weighted Average Interest Rates (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Amount	$ 222,636	$ 198,628
Century Trust [Member]
Debt Instrument [Line Items]
Amount	8,800	8,800
Weighted-Average Rate	10.88%	10.88%
Sequoia Trust I [Member]
Debt Instrument [Line Items]
Amount		2,065
Weighted-Average Rate		10.18%
United Statutory Trust III [Member]
Debt Instrument [Line Items]
Amount	20,619	20,619
Weighted-Average Rate	3.09%	3.10%
United Statutory Trust IV [Member]
Debt Instrument [Line Items]
Amount	25,774	25,774
Weighted-Average Rate	3.08%	3.09%
United Statutory Trust V [Member]
Debt Instrument [Line Items]
Amount	51,547	51,547
Weighted-Average Rate	1.79%	1.80%
United Statutory Trust VI [Member]
Debt Instrument [Line Items]
Amount	30,928	30,928
Weighted-Average Rate	1.54%	1.54%
Premier Statutory Trust II [Member]
Debt Instrument [Line Items]
Amount	6,186	6,186
Weighted-Average Rate	3.33%	3.34%
Premier Statutory Trust III [Member]
Debt Instrument [Line Items]
Amount	8,248	8,248
Weighted-Average Rate	1.98%	1.98%
Premier Statutory Trust IV [Member]
Debt Instrument [Line Items]
Amount	14,433	14,433
Weighted-Average Rate	1.80%	1.80%
Premier Statutory Trust V [Member]
Debt Instrument [Line Items]
Amount	10,310	10,310
Weighted-Average Rate	1.85%	1.85%
Centra Statutory Trust I [Member]
Debt Instrument [Line Items]
Amount	9,915	9,859
Weighted-Average Rate	2.54%	2.54%
Centra Statutory Trust II [Member]
Debt Instrument [Line Items]
Amount	9,915	9,859
Weighted-Average Rate	1.88%	1.89%
Virginia Commerce Trust II [Member]
Debt Instrument [Line Items]
Amount	11,323
Weighted-Average Rate	3.63%
Virginia Commerce Trust III [Member]
Debt Instrument [Line Items]
Amount	$ 14,638
Weighted-Average Rate	1.65%