Variable Interest Entities (Details) (USD $) Share data in Thousands, unless otherwise specified	12 Months Ended						12 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 Total Variable Interest Entities	Dec. 31, 2011 Total Variable Interest Entities	Dec. 31, 2012 Original Joint Venture Agreement / MarkWest Utica EMG	Dec. 31, 2009 MarkWest Pioneer	Dec. 31, 2012 MarkWest Pioneer item	Dec. 31, 2011 MarkWest Liberty Midstream
Variable interest entities
Percentage of ownership interest held in joint venture							60.00%
Initial contribution requirements by non-controlling interest in exchange for a ownership interest							$ 350,000,000
Noncontrolling interest owners total contribution							500,000,000
Percentage of ownership interest held by non-controlling interest							40.00%	50.00%
Percentage of capital required by noncontrolling owners after initial contribution							40.00%
Minimum Percentage of ownership interest held by non-controlling interest							30.00%
Percentage of capital contribution to maintain ownership percentage							30.00%
Percentage of initial capital funding of VIE							60.00%
Percentage of capital contribution after non-controlling interest minimum required contribution							100.00%
Percentage of additional capital contribution to increase ownership percentage							100.00%
Noncontrolling interest owners actual contribution							264,800,000
Length of FERC-regulated pipeline (in miles)									50
Takeaway capacity of Arkoma Connector Pipeline (in dekatherms per day)									638,000
Threshold above which entity is obligated to fund all capital expenditures								125,000,000
Percentage of non-controlling interest acquired by the Partnership										49.00%
Non-controlling interest purchase consideration, cash paid										994,000,000
Issuance of Class B units (in units)										19,954
ASSETS
Cash and cash equivalents	347,899,000	117,016,000	67,450,000	97,752,000	33,727,000	2,684,000	31,584,000		2,143,000
Restricted cash	25,500,000	26,193,000			500,000	0	500,000
Receivables, net	198,769,000	226,561,000			1,591,000	1,569,000	403,000		1,188,000
Other current assets	35,053,000	11,748,000			264,000	169,000	82,000		182,000
Property, plant and equipment, net	5,075,628,000	2,864,307,000			543,427,000		407,418,000		136,009,000
Accumulated depreciation	624,548,000	438,062,000			24,636,000	15,551,000	2,787,000		21,849,000
Other long-term assets	2,242,000	1,595,000			102,000	102,000			102,000
Total assets	6,835,716,000	4,070,425,000	3,333,362,000		579,611,000		439,987,000		139,624,000
LIABILITIES
Accounts payable	320,645,000	179,871,000			73,883,000	96,000	73,865,000		18,000
Accrued liabilities	391,352,000	171,451,000			110,746,000	1,144,000	109,572,000		1,174,000
Other long-term liabilities	134,340,000	121,356,000			79,000	73,000			79,000
Total liabilities					184,708,000		183,437,000		1,271,000
Capital expenditures	1,951,427,000	551,281,000	458,668,000
Summary of net income (loss) attributable to the entity and transfers to the non-controlling interest
Net income (loss) attributable to the Partnership	220,402,000	60,695,000	467,000
Transfers to the non-controlling interests:
Decrease in common unit equity for 2011 acquisition of equity interest in MarkWest Liberty Midstream, net of $51,321 income tax benefit		(1,194,865,000)
Tax benefit resulting from change in equity due to purchase of additional interest in consolidated subsidiary		51,321,000
Decrease in common unit equity for transaction costs related to 2011 acquisition of equity interest in MarkWest Liberty Midstream		(3,600,000)
Net (loss) income attributable to the Partnership and transfers to the non-controlling interest		$ (1,137,770,000)