Reconciliation of Segment Information, Total Assets (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Assets:
Assets	$ 12,593,194	$ 12,670,909	$ 12,595,611
Items excluded from segment assets:
Recoverable Income Taxes	6,602	0
Total Reportable Business Segments
Assets:
Assets	11,578,492		11,471,162
Coal
Assets:
Assets	5,698,504		5,360,577
Gas
Assets:
Assets	5,879,988		6,110,585
All Other Segments [Member]
Assets:
Assets	358,663		358,746
Excludes Gas Segment [Member]
Items excluded from segment assets:
Cash and Other Investments	23,669		121,807
Intersegment Elimination [Member]
Assets:
Assets	656,039		765,703
Items excluded from segment assets:
Recoverable Income Taxes	6,602		0
Deferred Tax Assets	585,829		596,657
Bond Issuance Costs	$ 39,939		$ 47,239