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, 2009	$ 1,309,553	$ 1,026,814		$ 282,739
Balance (in units) at Dec. 31, 2009		66,275
Increase (Decrease) in Equity
Issuance of units in public equity offerings, net of offering costs	142,255	142,255
Issuance of units in public equity offerings, net of offering costs (in units)		4,887
Distributions paid	(187,208)	(181,058)		(6,150)
Contributions from non-controlling interest	158,293			158,293
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
Deferred income tax impact from changes in equity	(7,614)	(7,614)
Net income	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
Increase (Decrease) in Equity
Issuance of units in public equity offerings, net of offering costs	1,095,488	1,095,488
Issuance of units in public equity offerings, net of offering costs (in units)		23,225
Distributions paid	(285,285)	(218,398)		(66,887)
Contributions from non-controlling interest	126,392			126,392
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
Deferred income tax impact from changes in equity	(62,227)	(62,227)
Issuance of Class B units	752,531		752,531
Issuance of Class B units (in units)			19,954
Purchase of non-controlling interest of MarkWest Liberty M&R, net of tax benefit	(1,698,810)	(1,198,465)		(500,345)
Net income	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
Increase (Decrease) in Equity
Issuance of units in public equity offerings, net of offering costs	1,634,081	1,634,081
Issuance of units in public equity offerings, net of offering costs (in units)		32,308
Distributions paid	(345,854)	(339,967)		(5,887)
Contributions from non-controlling interest	265,620			265,620
Share-based compensation activity	6,548	6,548
Share-based compensation activity (in units)		246
Excess tax benefits related to share-based compensation	907	907
Deferred income tax impact from changes in equity	(66,566)	(66,566)
Net income	218,788	220,402		(1,614)
Balance at Dec. 31, 2012	$ 3,215,591	$ 2,134,714	$ 752,531	$ 328,346
Balance (in units) at Dec. 31, 2012		127,494	19,954