Reconciliation of Segment Information, Total Assets (Details) (USD $)	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Assets:
Assets	$ 12,525,700,000	$ 12,070,610,000	$ 7,775,401,000
Items excluded from segment assets:
Recoverable Income Taxes	0	32,528,000
Deferred Tax Assets	1,848,582,000	1,960,332,000
Total Property, Plant and Equipment-Net	9,326,416,000	10,129,251,000	6,124,290,000
Total Reportable Business Segments
Assets:
Assets	11,436,808,000	10,972,676,000	6,894,003,000
Coal Segment [Member]
Assets:
Assets	5,253,226,000	5,056,583,000	4,722,508,000
Gas
Assets:
Assets	6,183,582,000	5,916,093,000	2,171,495,000
All Other Segments [Member]
Assets:
Assets	351,370,000	337,855,000	317,004,000
Excludes Gas Segment [Member]
Items excluded from segment assets:
Cash and Other Investments	39,655,000	16,836,000	65,025,000
Corporate Elimination [Member]
Assets:
Assets	737,522,000	760,079,000	564,394,000
Items excluded from segment assets:
Recoverable Income Taxes	0	32,528,000	0
Deferred Tax Assets	648,807,000	659,017,000	498,680,000
Bond Issuance Costs	49,060,000	51,698,000	689,000
UNITED STATES
Segment Reporting, Asset Reconciling Item [Line Items]
Disclosure of Long-Live Assets by Geographic Region	9,294,046,000	10,095,851,000	6,090,703,000
CANADA
Segment Reporting, Asset Reconciling Item [Line Items]
Disclosure of Long-Live Assets by Geographic Region	$ 32,370,000	$ 33,400,000	$ 33,587,000