Reconciliation of Segment Information, Total Assets (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Assets:
Assets	$ 12,737,072	$ 12,670,909	$ 12,556,594
Total Reportable Business Segments
Assets:
Assets	11,787,041		11,465,377
Coal
Assets:
Assets	5,792,969		5,594,926
Gas
Assets:
Assets	5,994,072		5,870,451
All Other Segments [Member]
Assets:
Assets	356,848		376,400
Excludes Gas Segment [Member]
Items excluded from segment assets:
Cash and Other Investments	17,988		40,331
Intersegment Elimination [Member]
Assets:
Assets	593,183		714,817
Items excluded from segment assets:
Recoverable Income Taxes	0		12,132
Deferred Tax Assets	538,930		618,742
Bond Issuance Costs	$ 36,265		$ 43,612