Acquisitions (Details 2) (USD $)	3 Months Ended				3 Months Ended								12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2011 Previously reported/Preliminary	Mar. 31, 2012 Western Coal Corp	Mar. 31, 2011 Western Coal Corp	Apr. 02, 2011 Western Coal Corp	Apr. 02, 2011 Western Coal Corp Common stock of Walter Energy	Apr. 02, 2011 Western Coal Corp Stock options issued and warrants assumed	Mar. 31, 2012 Western Coal Corp Canadian and U.K. operations	Mar. 31, 2012 Western Coal Corp U.S. Operations	Dec. 31, 2011 Western Coal Corp Previously reported/Preliminary	Dec. 31, 2011 Western Coal Corp Adjustments Purchase price allocation adjustments	Mar. 31, 2012 Western Coal Corp Adjustments Purchase price allocation adjustments
Acquisitions
Net income	$ 40,616,000	$ 81,813,000											$ 14,400,000
ASSETS
Inventories	309,742,000		240,437,000	242,607,000
Other current assets	39,242,000		45,649,000	45,627,000
Mineral interests, net	3,034,630,000		3,056,258,000	2,946,113,000
Property, plant and equipment, net	1,678,957,000		1,631,333,000	1,637,182,000
Goodwill	1,066,754,000		1,066,754,000	1,124,597,000
LIABILITIES AND STOCKHOLDERS' EQUITY
Other current liabilities	55,953,000		63,757,000	59,827,000
Deferred income taxes	1,018,333,000		1,029,336,000	1,003,383,000
Retained earnings	777,103,000		744,939,000	730,517,000									13,300,000
Purchase consideration:
Cash consideration							2,173,080,000
Fair value of shares of common stock issued or stock options issued and warrants								1,224,126,000	34,765,000
Fair value of consideration transferred							3,431,971,000
Business Combination Step Acquisition Equity Interest in Acquiree Fair Value							314,231,000
Total consideration							3,746,202,000
Fair value of assets acquired and liabilities assumed
Cash and cash equivalents					34,065,000							34,065,000
Receivables					163,668,000							163,668,000
Inventories					121,229,000							121,229,000
Other current assets					86,498,000							86,475,000		23,000
Mineral interests					3,086,000,000							2,992,000,000		94,000,000
Property, plant and equipment					554,192,000							560,894,000		(6,702,000)
Goodwill					1,065,040,000					992,400,000	72,600,000	1,122,884,000		(57,844,000)
Other long-term assets					54,150,000							54,150,000
Total assets					5,164,842,000							5,135,365,000		29,477,000
Accounts payable and accrued liabilities					184,983,000							184,983,000
Other current liabilities					86,105,000							82,175,000		3,930,000
Deferred tax liability					1,046,708,000							1,021,161,000		25,547,000
Other long-term liabilities					100,844,000							100,844,000
Total liabilities					1,418,640,000							1,389,163,000		29,477,000
Net assets acquired					3,746,202,000							3,746,202,000
Acquisition and integration related costs (included in Selling, general and administrative expenses at the end of the period)						9,900,000
Supplemental Pro forma Combined Results
Revenue					632,942,000
Net income					$ 131,008,000