Acquisitions - Additional Information (details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012		Sep. 30, 2013	Sep. 30, 2012
Business Acquisition [Line Items]
Revenues	$ 278,001	$ 234,734	[1]	$ 762,874	$ 679,720	[1]
Operating expenses	188,499	173,422	[1]	541,395	491,907	[1]
Anadarko-Operated Marcellus Interest [Member] | Western Gas Partners [Member]
Business Acquisition [Line Items]
Revenues				8,100
Operating expenses				500
Purchase price adjustment	134,645			134,645
Anadarko-Operated Marcellus Interest [Member] | Purchase Price Allocation Adjustments [Member] | Western Gas Partners [Member]
Business Acquisition [Line Items]
Purchase price adjustment	$ 1,100			$ 1,100

[1] Financial information has been recast to include the financial position and results attributable to the Non-Operated Marcellus Interest. See Note 2.