Reconciliation of Segment Information, Total Assets (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Assets:
Total Assets	$ 11,718,935	$ 11,393,667	$ 12,737,072
Items excluded from segment assets:
Recoverable Income Taxes	41,971	10,705
Total Reportable Segments [Member]
Assets:
Total Assets	11,021,287		10,112,423
Gas
Assets:
Total Assets	6,901,696		5,994,072
Coal Segment [Member]
Assets:
Total Assets	4,119,591		4,118,351
Other Segments [Member]
Assets:
Total Assets	295,279		356,731
Excludes Gas Segment [Member]
Items excluded from segment assets:
Cash and Other Investments	193,325		17,966
Intersegment Eliminations [Member]
Assets:
Total Assets	402,369		2,267,918
Items excluded from segment assets:
Recoverable Income Taxes	41,971		0
Deferred Tax Assets	127,731		25,585
Bond Issuance Costs	39,342		36,266
Assets of Disposal Group, Including Discontinued Operation	$ 0		$ 2,188,101