Condensed Consolidated Statements of Cash Flows (USD $) In Thousands	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010
Cash flows from operating activities:
Net income	$ 14,534	$ 92,972
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	71,565	58,005
Amortization of intangible assets	21,647	20,386
Loss on redemption of debt	43,328
Amortization of deferred financing costs and discount	2,871	4,892
Accretion of asset retirement obligations	377	212
Amortization of deferred contract cost	156	156
Phantom unit compensation expense	8,093	8,253
Loss (earnings) of unconsolidated affiliate	755	(1,517)
Distribution from unconsolidated affiliate	300	1,155
Unrealized loss (gain) on derivative instruments	26,265	(62,987)
Loss on disposal of property, plant and equipment	4,472	179
Deferred income taxes	(3,567)	13,726
Changes in operating assets and liabilities, net of working capital acquired:
Receivables	(9,215)	(16,829)
Inventories	(6,326)	(378)
Other current assets	526	4,812
Accounts payable and accrued liabilities	20,473	5,803
Other long-term assets	(384)	(618)
Other long-term liabilities	10,494	2,414
Net cash 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
Cash paid for taxes related to net settlement of share-based payment awards	(6,269)	(3,730)
Excess tax benefits related to share-based compensation	1,096	97
Payment of distributions to common unitholders	(100,058)	(88,858)
Payment of distributions to non-controlling interest	(34,531)	(2,715)
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