Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended								3 Months Ended												12 Months Ended
	Jan. 31, 2012	Dec. 31, 2011	Oct. 31, 2011	Jul. 31, 2011	Jan. 31, 2011	Apr. 30, 2010	Aug. 31, 2009	Jun. 30, 2009	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, 2009	Sep. 30, 2009	Jun. 30, 2009	Mar. 31, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Distributions of Available Cash
Declaration Date									January 26, 2012	October 18, 2011	July 21, 2011	April 21, 2011	January 27, 2011	October 27, 2010	July 22, 2010	April 22, 2010	January 26, 2010	October 22, 2009	July 23, 2009	April 23, 2009
Record Date									February 6, 2012	November 7, 2011	August 1, 2011	May 2, 2011	February 7, 2011	November 8, 2010	August 2, 2010	May 3, 2010	February 5, 2010	November 2, 2009	August 3, 2009	May 4, 2009
Payment Date									February 14, 2012	November 14, 2011	August 12, 2011	May 13, 2011	February 14, 2011	November 12, 2010	August 13, 2010	May 14, 2010	February 12, 2010	November 13, 2009	August 14, 2009	May 15, 2009
Distribution per common unit (in dollars per unit)									$ 0.76	$ 0.73	$ 0.70	$ 0.67	$ 0.65	$ 0.64	$ 0.64	$ 0.64	$ 0.64	$ 0.64	$ 0.64	$ 0.64	$ 2.75	$ 2.56	$ 2.56
Equity Offerings
Issuance of units in public offerings, net of offering costs (in units)	700,000	10,000,000	5,750,000	4,000,000	3,450,000	4,900,000	6,030,000	3,340,000
Net proceeds from public offering after deducting underwriters fees and other third-party expenses	$ 38,000	$ 521,000	$ 251,000	$ 185,100	$ 138,200	$ 142,300	$ 120,900	$ 57,700													$ 1,095,488	$ 142,255	$ 178,565
Percentage of non-controlling interest acquired by the Partnership		49.00%							49.00%												49.00%
MarkWest Liberty Midstream
Class B Units Issuance
Non-controlling interest purchase consideration, shares																					19,954,000
Equity Offerings
Percentage of non-controlling interest acquired by the Partnership		49.00%							49.00%												49.00%
Class B Units
Class B Units Issuance
Non-controlling interest purchase consideration, shares																					19,954,000
Partner's units conversion basis																					1
Number of installments in which partner's units are converted																					5
Number of anniversaries of July 31, 2013 on which Class B units will be converted into common units																					4
Limited partners' capital account, units authorized for distribution		2,500,000							2,500,000												2,500,000
Minimum
Class B Units Issuance
Number of offerings in any twelve month period																					1
Maximum
Equity
Period after the end of each quarter within which available cash is distributed to unitholders (in days)																					45
Period for which cash reserves are created for distribution to unitholders (in quarters)																					4
Class B Units Issuance
Participation in underwritten offerings as percentage of common stock offered																					20.00%
Number of underwritten offerings																					3
Maximum | Class B Units
Class B Units Issuance
Partner's unit voting rights based on specified percentage of total common units																					0.05