United Bankshares, Inc. (Parent Company Only) Financial Information (Details 2) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating Activities
Net income	$ 20,257,000	$ 20,016,000	$ 17,452,000	$ 17,885,000	$ 19,290,000	$ 17,316,000	$ 17,919,000	$ 17,422,000	$ 75,610,000	$ 71,947,000	$ 67,298,000
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of net periodic pension costs									(912,000)	(1,467,000)	(4,470,000)
Stock-based compensation									1,133,000	1,050,000	484,000
Net (gain) loss on securities transactions									(18,838,000)	(7,809,000)	(14,705,000)
Net Cash Provided by Operating Activities									118,567,000	100,824,000	53,657,000
Investing Activities
Net Cash (Used in) Provided by Investing Activities									158,337,000	631,923,000	545,161,000
Financing Activities
Cash dividends paid									(54,344,000)	(52,257,000)	(50,383,000)
Acquisition of treasury stock									(18,000)	(11,000)	(1,180,000)
Proceeds from sale of treasury stock from deferred compensation plan									74,000	520,000	536,000
Excess tax benefits from stock-based compensation arrangements									3,114,000	459,000	273,000
Proceeds from exercise of stock options									542,000	2,492,000	724,000
Net Cash Used in Financing Activities									(102,290,000)	(721,125,000)	(362,585,000)
Increase/(Decrease) in cash and cash equivalents									174,614,000	11,622,000	236,233,000
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR				461,389,000				449,767,000	461,389,000	449,767,000	213,534,000
CASH AND CASH EQUIVALENTS AT END OF YEAR	636,003,000				461,389,000				636,003,000	461,389,000	449,767,000
United Bankshares
Operating Activities
Net income									75,610,000	71,947,000	67,298,000
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed net income of subsidiaries									(11,835,000)	(13,348,000)	(18,920,000)
Amortization of net periodic pension costs									76,000	117,000	257,000
Stock-based compensation									1,133,000	1,050,000	484,000
Net (gain) loss on securities transactions									(12,000)	(54,000)	179,000
Net change in other assets and liabilities									(6,895,000)	(1,940,000)	(1,376,000)
Net Cash Provided by Operating Activities									58,077,000	57,772,000	47,922,000
Investing Activities
Net proceeds from (purchases of) sales of securities									4,854,000	(240,000)	243,000
Net cash paid in acquisition of subsidiary									8,576,000	0	0
Change in other investment securities									1,376,000	37,000	52,000
Net Cash (Used in) Provided by Investing Activities									(2,346,000)	(203,000)	295,000
Financing Activities
Cash dividends paid									(54,344,000)	(52,257,000)	(50,383,000)
Acquisition of treasury stock									(18,000)	(11,000)	(1,180,000)
Proceeds from sale of treasury stock from deferred compensation plan									74,000	520,000	536,000
Excess tax benefits from stock-based compensation arrangements									3,114,000	459,000	273,000
Proceeds from exercise of stock options									542,000	2,492,000	724,000
Net Cash Used in Financing Activities									(50,632,000)	(48,797,000)	(50,030,000)
Increase/(Decrease) in cash and cash equivalents									5,099,000	8,772,000	(1,813,000)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR				26,397,000				17,625,000	26,397,000	17,625,000	19,438,000
CASH AND CASH EQUIVALENTS AT END OF YEAR	$ 31,496,000				$ 26,397,000				$ 31,496,000	$ 26,397,000	$ 17,625,000