CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (USD $) In Thousands, unless otherwise specified	Total	Common Units	Class B Units	Non-controlling Interest
Balance at Dec. 31, 2008	$ 1,148,155	$ 1,144,854		$ 3,301
Balance (in units) at Dec. 31, 2008		56,640
Increase (Decrease) in Equity
Share-based compensation activity	5,204	5,204
Share-based compensation activity (in units)		275
Distributions paid	(155,462)	(155,307)		(155)
Issuance of units in public offering, net of offering costs	178,565	178,565
Issuance of units in public offerings, net of offering costs (in units)		9,360
Contributions to MarkWest Liberty Midstream joint venture, net	194,536	(5,464)		200,000
Proceeds from sale of equity interest in joint venture, net	60,654	(1,846)		62,500
Transfer to non-controlling interest from sale of equity interest in joint venture, net of tax	1,491	(10,288)		11,779
Deferred income tax impact from changes in equity	(10,236)	(10,236)
Net income (loss)	(113,354)	(118,668)		5,314
Balance at Dec. 31, 2009	1,309,553	1,026,814		282,739
Balance (in units) at Dec. 31, 2009		66,275
Increase (Decrease) in Equity
Share-based compensation activity	12,087	12,087
Share-based compensation activity (in units)		278
Excess tax benefits related to share-based compensation	98	98
Distributions paid	(187,208)	(181,058)		(6,150)
Issuance of units in public offering, net of offering costs	142,255	142,255
Issuance of units in public offerings, net of offering costs (in units)		4,887
Contributions to MarkWest Liberty Midstream joint venture, net	158,293			158,293
Deferred income tax impact from changes in equity	(7,614)	(7,614)
Net income (loss)	31,102	467		30,635
Balance at Dec. 31, 2010	1,458,566	993,049		465,517
Balance (in units) at Dec. 31, 2010		71,440	0
Increase (Decrease) in Equity
Share-based compensation activity	8,083	8,083
Share-based compensation activity (in units)		275
Excess tax benefits related to share-based compensation	1,084	1,084
Distributions paid	(285,285)	(218,398)		(66,887)
Issuance of units in public offering, net of offering costs	1,095,488	1,095,488
Issuance of units in public offerings, net of offering costs (in units)		23,225
Issuance of Class B units	752,531		752,531
Issuance of Class B units (in units)			19,954
Contributions to MarkWest Liberty Midstream joint venture, net	126,392			126,392
Purchase of non-controlling interest of MarkWest Liberty M&R, net of tax benefit	(1,698,810)	(1,198,465)		(500,345)
Deferred income tax impact from changes in equity	(62,227)	(62,227)
Net income (loss)	106,245	60,695		45,550
Balance at Dec. 31, 2011	$ 1,502,067	$ 679,309	$ 752,531	$ 70,227
Balance (in units) at Dec. 31, 2011		94,940	19,954