Business Combinations (Narrative) (Details) (USD $) Share data in Thousands, unless otherwise specified	12 Months Ended			0 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jun. 15, 2011 Number Of Mining Complexes - Appalachia [Member]	Jun. 15, 2011 Number Of Mining Complexes - Illinois [Member]	Oct. 01, 2009 Jacobs Ranch Mining Operations [Member]
Business Acquisition [Line Items]
Number of mining complexes operated				12	one
Number of mining complexes being developed				one
Proceeds from the issuance of common stock, net	$ 1,267,933,000		$ 326,452,000
Number of shares sold	48,705		19,550
Proceeds from issuance of aggregate principal amount of senior notes	2,000,000,000	500,000,000	584,784,000
Revenues related to the acquired operations included in the statement of income since acquisition	606,900,000
Income before income taxes related to the acquired operations included in the statement of income since acquisition		14,600,000
Restricted cash	10,300,000
Business Acquisition, Cost of Acquired Entity, Purchase Price						$ 768,800,000
Tons Of Coal						345,000,000