Supplemental Condensed Consolidating Financial Information (Details 3) (USD $) In Thousands	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010
Condensed Consolidating Statements of Cash Flows
Net cash (used in) provided by operating activities	$ 206,364	$ 130,636
Cash flows from investing activities:
Capital expenditures	(234,116)	(253,260)
Acquisitions	(230,728)
Proceeds from disposal of property, plant and equipment	2,795	417
Net cash used in investing activities	(462,049)	(252,843)
Cash flows from financing activities:
Proceeds from revolving credit facility	781,700	213,404
Payments of revolving credit facility	(535,200)	(221,204)
Proceeds from long-term debt	499,000
Payments of long-term debt	(437,848)
Payments of premiums on redemption of long-term debt	(39,520)
Payments for debt issuance costs, deferred financing costs and registration costs	(6,747)
Contributions to MarkWest Liberty Midstream joint venture	24,400	120,557
Payments of SMR liability	(916)	(480)
Proceeds from public offering, net	138,163	142,255
Share-based payment activity	(5,173)	(3,633)
Payment of distributions	(134,589)	(91,573)
Net cash provided by financing activities	283,270	159,326
Net increase in cash	27,585	37,119
Cash and cash equivalents at beginning of year	67,450	97,752
Cash and cash equivalents at end of period	95,035	134,871
Parent
Condensed Consolidating Statements of Cash Flows
Net cash (used in) provided by operating activities	(66,828)	(48,230)
Cash flows from investing activities:
Capital expenditures	(610)	(282)
Equity investments	(21,556)	(20,540)
Distributions from consolidated affiliates	27,208	25,238
Collection of (Investment in) intercompany notes, net	(17,450)	550
Net cash used in investing activities	(12,408)	4,966
Cash flows from financing activities:
Proceeds from revolving credit facility	781,700	213,404
Payments of revolving credit facility	(535,200)	(221,204)
Proceeds from long-term debt	499,000
Payments of long-term debt	(437,848)
Payments of premiums on redemption of long-term debt	(39,520)
Payments for debt issuance costs, deferred financing costs and registration costs	(6,747)
Proceeds from public offering, net	138,163	142,255
Share-based payment activity	(6,269)	(3,730)
Payment of distributions	(100,058)	(88,858)
Intercompany advances, net	(213,982)	1,397
Net cash provided by financing activities	79,239	43,264
Net increase in cash	3
Cash and cash equivalents at end of period	3
Guarantor Subsidiaries
Condensed Consolidating Statements of Cash Flows
Net cash (used in) provided by operating activities	189,342	158,451
Cash flows from investing activities:
Capital expenditures	(58,715)	(76,186)
Acquisitions	(230,728)
Equity investments	(130,361)	(101,629)
Distributions from consolidated affiliates	35,147	6,395
Proceeds from disposal of property, plant and equipment	89	417
Net cash used in investing activities	(384,568)	(171,003)
Cash flows from financing activities:
Payments of intercompany notes receivable, net	(5,550)	(550)
Contributions from parent, net	21,556	20,540
Payments of SMR liability	(916)	(480)
Share-based payment activity	1,096	97
Payment of distributions	(27,208)	(25,238)
Intercompany advances, net	213,982	(1,397)
Net cash provided by financing activities	202,960	(7,028)
Net increase in cash	7,734	(19,580)
Cash and cash equivalents at beginning of year	63,850	74,448
Cash and cash equivalents at end of period	71,584	54,868
Non-Guarantor Subsidiaries
Condensed Consolidating Statements of Cash Flows
Net cash (used in) provided by operating activities	87,113	23,266
Cash flows from investing activities:
Capital expenditures	(179,302)	(179,643)
Proceeds from disposal of property, plant and equipment	3,954
Net cash used in investing activities	(175,348)	(179,643)
Cash flows from financing activities:
Payments of intercompany notes receivable, net	23,000
Contributions to joint ventures, net	154,761	222,186
Payment of distributions	(69,678)	(9,110)
Net cash provided by financing activities	108,083	213,076
Net increase in cash	19,848	56,699
Cash and cash equivalents at beginning of year	3,600	23,304
Cash and cash equivalents at end of period	23,448	80,003
Consolidating Adjustments
Condensed Consolidating Statements of Cash Flows
Net cash (used in) provided by operating activities	(3,263)	(2,851)
Cash flows from investing activities:
Capital expenditures	4,511	2,851
Equity investments	151,917	122,169
Distributions from consolidated affiliates	(62,355)	(31,633)
Collection of (Investment in) intercompany notes, net	17,450	(550)
Proceeds from disposal of property, plant and equipment	(1,248)
Net cash used in investing activities	110,275	92,837
Cash flows from financing activities:
Payments of intercompany notes receivable, net	(17,450)	550
Contributions from parent, net	(21,556)	(20,540)
Contributions to joint ventures, net	(130,361)	(101,629)
Payment of distributions	62,355	31,633
Net cash provided by financing activities	$ (107,012)	$ (89,986)