United Bankshares, Inc. (Parent Company Only) Financial Information (Condensed Statements of Income) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Other Items Net Interest And Other Financial Income [Line Items]
Net interest income	$ 69,672	$ 70,014	$ 69,055	$ 68,966	$ 71,992	$ 70,752	$ 58,621	$ 59,363	$ 277,707	$ 260,728	$ 238,186
Income Before Income Taxes and Equity in Undistributed Net Income of Subsidiaries									121,481	110,376	104,404
Applicable income tax benefit	9,983	9,099	9,905	9,887	9,312	9,204	8,026	8,224	38,874	34,766	32,457
Net income	21,214	19,334	21,049	21,010	20,257	20,016	17,452	17,885	82,607	75,610	71,947
United Bankshares [Member]
Other Items Net Interest And Other Financial Income [Line Items]
Dividends from banking subsidiaries									67,600	68,323	63,138
Net interest income									119	417	288
Bank subsidiaries									15,854	13,000	11,077
Nonbank subsidiaries									27	27	27
Other income									105	414	450
Total Income									83,705	82,181	74,980
Operating expenses									20,879	20,431	18,402
Income Before Income Taxes and Equity in Undistributed Net Income of Subsidiaries									62,826	61,750	56,578
Applicable income tax benefit									(1,487)	(2,025)	(2,022)
Income Before Equity in Undistributed Net Income of Subsidiaries									64,313	63,775	58,600
Bank subsidiaries									18,186	11,710	13,223
Nonbank subsidiaries									108	125	124
Net income									$ 82,607	$ 75,610	$ 71,947