Dispositions (Details) a in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) a	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Property, Plant, and Equipment	$ 226,820	$ 982,934	$ 1,459,452
Gas and Oil Area, Undeveloped, Gross | a	34,467
Gain (Loss) on Disposition of Assets	$ 19,498	74,173	43,198
Working Capital Adjustment		6,258
Industrial Supplies Subsidiary [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Property, Plant, and Equipment			51,000
cash flow from investing activities			44,035
Gain (Loss) on Disposition of Assets			30,845
Western Allegheny Energy [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (Loss) on Disposition of Assets	48,468
Business Acquisition, Cost of Acquired Entity, Transaction Costs		76,297
Certain Loans Acquired in Transfer, Nonaccretable Difference		2,136
Accumulated Distributions in Excess of Net Income		10,780
Columbia Energy Ventures [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Property, Plant, and Equipment	9,000	50,969
Business Acquisition, Cost of Acquired Entity, Transaction Costs			$ 96,106
Other Liabilities, Current	3,947	8,349
Other Long-term Debt, Noncurrent	$ 26,461	$ 29,333