Reconciliation of Segment Information, Total Assets (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Assets:
Total Assets	$ 11,566,647	$ 11,393,667	$ 12,752,928
Items excluded from segment assets:
Recoverable Income Taxes	47,060	10,705
Total Reportable Segments [Member]
Assets:
Total Assets	10,907,870		10,382,403
Gas
Assets:
Total Assets	6,797,166		6,170,531
Coal Segment [Member]
Assets:
Total Assets	4,110,704		4,211,872
Other Segments [Member]
Assets:
Total Assets	297,044		363,819
Excludes Gas Segment [Member]
Items excluded from segment assets:
Cash and Other Investments	136,266		45,885
Intersegment Eliminations [Member]
Assets:
Total Assets	361,733		2,006,706
Items excluded from segment assets:
Recoverable Income Taxes	47,060		1,930
Deferred Tax Assets	137,716		84,402
Bond Issuance Costs	40,691		38,102
Assets of Disposal Group, Including Discontinued Operation	$ 0		$ 1,836,387