Variable Interest Entities (Details) (USD $)							3 Months Ended			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2013 Total Variable Interest Entities	Dec. 31, 2012 Total Variable Interest Entities	Mar. 31, 2013 MarkWest Utica EMG	Dec. 31, 2012 MarkWest Utica EMG	Jan. 02, 2012 MarkWest Utica EMG	Mar. 31, 2013 MarkWest Pioneer	Dec. 31, 2012 MarkWest Pioneer
Variable interest entities
Noncontrolling interest owners total funding commitment							$ 950,000,000		$ 500,000,000
Noncontrolling interest owners initial funding commitment to VIE							750,000,000
Accrual of preference amount to EMG Utica's investment balance during the period							0
Percentage of initial capital funding of VIE							60.00%
Percentage of capital required by reporting entity after non-controlling interest minimum contribution							100.00%
Aggregate contributions to VIE threshold							2,000,000,000
Percentage of ownership interest in joint venture maximum							70.00%
Minimum Percentage of ownership interest held by non-controlling interest							30.00%
Maximum percentage of capital contribution to maintain ownership percentage							10.00%
Noncontrolling interest owners capital contribution preference threshold							500,000,000
Minimum percentage of ownership interest held in joint venture							51.00%
Percentage of available cash to be received by the Partnership							60.00%
Maximum temporary funding by reporting entity							150,000,000
Temporary capital contributed by partnership							76,200,000
Distribution received as reimbursement for temporary funding							61,200,000
Capital contributed by partnership retained							15,000,000
Percentage of ownership interest held in joint venture							60.00%			50.00%
Percentage of ownership interest held by non-controlling interest										50.00%
ASSETS
Cash and cash equivalents	654,170,000	347,899,000	350,634,000	117,016,000	150,728,000	33,727,000	148,067,000	31,584,000		2,661,000	2,143,000
Restricted cash	500,000	25,500,000			500,000	500,000	500,000	500,000
Receivables, net	212,957,000	198,769,000			1,248,000	1,591,000	160,000	403,000		1,088,000	1,188,000
Other current assets	24,070,000	35,053,000			1,594,000	264,000	1,463,000	82,000		131,000	182,000
Property, plant and equipment, net	5,691,947,000	5,075,628,000			857,732,000	543,427,000	723,303,000	407,418,000		134,429,000	136,009,000
Accumulated depreciation	688,724,000	624,548,000			30,670,000	24,636,000	7,241,000	2,787,000		23,429,000	21,849,000
Other long-term assets	2,209,000	2,242,000			102,000	102,000				102,000	102,000
Total assets	7,720,554,000	6,835,716,000			1,011,904,000	579,611,000	873,493,000	439,987,000		138,411,000	139,624,000
LIABILITIES
Accounts payable	300,056,000	320,645,000			81,992,000	73,883,000	81,978,000	73,865,000		14,000	18,000
Accrued liabilities	435,483,000	391,352,000			136,794,000	110,746,000	135,677,000	109,572,000		1,117,000	1,174,000
Other long-term liabilities	137,856,000	134,340,000			80,000	79,000				80,000	79,000
Total liabilities	$ 218,866,000	$ 184,708,000			$ 218,866,000	$ 184,708,000	$ 217,655,000	$ 183,437,000		$ 1,211,000	$ 1,271,000