Acquisitions (Details) a in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) a	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Gas and Oil Area, Undeveloped, Gross | a	34,467
Payments to Acquire Property, Plant, and Equipment	$ 226,820	$ 982,934	$ 1,459,452
Columbia Energy Ventures [Member]
Business Acquisition [Line Items]
Business Acquisition, Cost of Acquired Entity, Transaction Costs			$ 96,106
Payments to Acquire Property, Plant, and Equipment	9,000	50,969
Other Liabilities, Current	3,947	8,349
Other Long-term Debt, Noncurrent	$ 26,461	$ 29,333