Acquisitions - Impact to Historical Consolidated Statement of Income Table (details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Prior Period Adjustments [Line Items]
Revenues	$ 229,747	$ 224,676	[1]
Net income	51,676	45,628	[1]
Scenario Previously Reported Member
Prior Period Adjustments [Line Items]
Revenues		212,242
Net income		40,140
Non-Operated Marcellus Interest [Member]
Prior Period Adjustments [Line Items]
Revenues		12,434
Net income		$ 5,488

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