United Bankshares, Inc. (Parent Company Only) Financial Information - Condensed Balance Sheets (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and due from banks	$ 175,713,000	$ 134,808,000
Securities available for sale	1,180,386,000	775,284,000
Securities held to maturity	36,784,000	38,293,000
Other investment securities	96,344,000	73,093,000
Other assets	402,256,000	322,634,000
TOTAL ASSETS	12,328,811,000	8,735,324,000
Accrued expenses and other liabilities	84,682,000	63,427,000
Shareholders' equity (including other accumulated comprehensive loss of $35,764 and $43,047 at December 31, 2014 and 2013, respectively)	1,656,160,000	1,041,732,000	992,251,000	968,844,000
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	12,328,811,000	8,735,324,000
United Bankshares [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and due from banks	38,984,000	43,613,000
Securities available for sale	2,132,000	1,925,000
Securities held to maturity	20,000	20,000
Other investment securities	102,000	125,000
Bank subsidiaries	1,769,962,000	1,139,786,000
Nonbank subsidiaries	6,436,000	6,396,000
Other assets	13,029,000	11,751,000
TOTAL ASSETS	1,830,665,000	1,203,616,000
Junior subordinated debentures of subsidiary trusts	128,868,000	128,868,000
Accrued expenses and other liabilities	45,637,000	33,016,000
Shareholders' equity (including other accumulated comprehensive loss of $35,764 and $43,047 at December 31, 2014 and 2013, respectively)	1,656,160,000	1,041,732,000
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 1,830,665,000	$ 1,203,616,000