Variable Interest Entities (Details) (USD $) Share data in Thousands, unless otherwise specified	3 Months Ended								12 Months Ended						1 Months Ended	9 Months Ended	12 Months Ended				12 Months Ended
	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, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2011 Total Variable Interest Entities	Dec. 31, 2010 Total Variable Interest Entities	Feb. 27, 2009 MarkWest Liberty Midstream	Dec. 31, 2009 MarkWest Liberty Midstream	Dec. 31, 2011 MarkWest Liberty Midstream	Dec. 31, 2010 MarkWest Liberty Midstream	Dec. 31, 2009 MarkWest Liberty Midstream	Mar. 01, 2009 MarkWest Liberty Midstream	Dec. 31, 2009 MarkWest Pioneer	Dec. 31, 2011 MarkWest Pioneer mile	Dec. 31, 2010 MarkWest Pioneer
Variable interest entities
MarkWest's percentage of ownership															60.00%		51.00%	60.00%	60.00%
Agreed-to value of contributed assets															$ 107,500,000
Cash contributed by M&R MWE Liberty, LLC in exchange for a minority ownership interest											60,654,000
Cash contributed by M&R MWE Liberty, LLC in exchange for a minority ownership interest															50,000,000	150,000,000	126,400,000	158,300,000
Funding commitment by M&R requirements after which time the Partnership agreed to fund the future capital requirements until each member's contributed capital was proportionate to its ownership interest															150,000,000
Percentage of non-controlling interest																40.00%	49.00%	40.00%	40.00%	40.00%	50.00%	50.00%	50.00%
Percentage of non-controlling interest acquired by the Partnership	49.00%								49.00%								49.00%
Special non-cash allocation of net income received by M & R																	1,300,000	11,400,000	3,400,000
MarkWest additional contributions to Liberty																	252,400,000	171,100,000	8,000,000
Non-controlling interest purchase consideration																	1,746,500,000
Non-controlling interest purchase consideration, cash paid																	994,000,000
Non-controlling interest purchase consideration, shares																	19,954
Non-controlling interest purchase consideration, value Class B units									752,531,000								752,500,000
Transaction costs related to acquisition of non-controlling interests																	3,600,000
Carrying value of non-controlling interest acquired																	500,300,000
Percentage of profit loss attributable to the entity																	100.00%
Net income (loss) of VIE attributable to non-controlling interests									45,550,000	30,635,000	5,314,000						44,000,000	27,900,000	6,300,000
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
ASSETS
Cash and cash equivalents	117,016,000				67,450,000				117,016,000	67,450,000	97,752,000	3,321,000	2,684,000	2,913,000									2,913,000
Receivables, net	226,561,000				179,209,000				226,561,000	179,209,000			1,569,000	43,783,000				42,181,000					1,602,000
Inventories	41,006,000				23,432,000				41,006,000	23,432,000			0	8,431,000				8,431,000
Other current assets	11,748,000				8,020,000				11,748,000	8,020,000			169,000	272,000				271,000					1,000
Property, plant and equipment, net	2,864,307,000				2,319,024,000				2,864,307,000	2,319,024,000				811,817,000				664,778,000					147,039,000
Accumulated depreciation	438,062,000				294,003,000				438,062,000	294,003,000			15,551,000	38,169,000				28,869,000					9,300,000
Restricted cash					28,001,000					28,001,000			0	28,001,000				28,001,000
Other long-term assets	1,595,000				1,486,000				1,595,000	1,486,000			102,000	383,000				281,000					102,000
Total assets	4,070,425,000				3,333,362,000				4,070,425,000	3,333,362,000	3,014,737,000			895,600,000				743,943,000					151,657,000
LIABILITIES
Accounts payable	179,871,000				122,473,000				179,871,000	122,473,000			96,000	5,945,000				5,945,000
Accrued liabilities	171,451,000				153,869,000				171,451,000	153,869,000			1,144,000	64,713,000				63,450,000					1,263,000
Other long-term liabilities	121,356,000				105,349,000				121,356,000	105,349,000			73,000	154,000				86,000					68,000
Total liabilities														70,812,000				69,481,000					1,331,000
Summary of net income (loss) attributable to the entity and transfers to the non-controlling interest
Net income (loss) attributable to the Partnership	(74,085,000)	140,312,000	78,497,000	(84,029,000)	(54,109,000)	(27,151,000)	60,217,000	21,510,000	60,695,000	467,000	(118,668,000)
Transfers to the non-controlling interest:
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)
Decrease in Partners' Capital common unit equity for transfer to non-controlling interest from 2009 sale of equity interest in MarkWest Pioneer, net of $1,491 income tax benefit											(10,288,000)
Tax benefit resulting from change in equity due to sale of interest in consolidated subsidiary											1,491,000
Decrease in Partners' Capital for transaction costs related to sale of equity interest in MarkWest Liberty Midstream and MarkWest Pioneer											(7,310,000)
Net (loss) income attributable to the Partnership and transfers to the non-controlling interest									$ (1,137,770,000)	$ 467,000	$ (136,266,000)