Variable Interest Entity and Equity Method Investment (Details) (USD $)	6 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2013 Variable Interest Entity MarkWest Utica EMG	Jan. 02, 2012 Variable Interest Entity MarkWest Utica EMG	Jun. 30, 2013 Equity Method Investment MarkWest Pioneer	Dec. 31, 2009 Equity Method Investment MarkWest Pioneer
Variable Interest Entities and Equity Method Investment
Noncontrolling interest owners total funding commitment	$ 950,000,000	$ 500,000,000
Noncontrolling interest owners initial funding commitment to VIE		750,000,000
Percentage of initial capital funding of VIE	60.00%
Accrual of preference amount to EMG Utica's investment balance during the period	6,400,000
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%	50.00%
Impairment of investment				39,200,000
Impairment of investment, net of provision for income tax				$ 35,400,000