Divestitures (Results of Discontinued Operations) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income from discontinued operations	$ 0	$ 79,404	$ 0	$ 101,816
Canyon Fuel [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, before Income Tax		3,429		32,166
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax		115,679		115,679
Less: income tax expense		39,704		46,029
Income from discontinued operations		79,404		101,816
Basic and diluted earnings per common share from discontinued operations		$ 0.37		$ 0.48
Canyon Fuel [Member] | Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenues		$ 45,763		$ 219,002