Acquisitions (Details) a in Thousands, $ in Thousands	1 Months Ended	12 Months Ended
	Jun. 30, 2013 USD ($) a well	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Acquisitions
Purchase price of acquisition allocated to undeveloped acreage		$ 158,405	$ 493,067	$ 95,861
Purchase price of acquisition allocated to acquired wells		$ 23,890	$ 231,322	90,390
Chesapeake
Acquisitions
Approximate area of acquisition (in acres) | a	99
Purchase price of acquisition	$ 114,200
Purchase price of acquisition allocated to undeveloped acreage	$ 57,200
Chesapeake | Marcellus
Acquisitions
Approximate area of acquisition (in acres) | a	67
Number of wells acquired | well	10
Portion unlikely to be developed (in acres) | a	42
Purchase price of acquisition allocated to undeveloped acreage				$ 41,900
Purchase price of acquisition allocated to acquired wells	$ 57,000