Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating segment results
Revenues	$ 2,016.9	$ 2,058.8	$ 1,981.1	$ 2,020.7	$ 2,229.8	$ 1,980.6	$ 1,961.7	$ 1,723.8	$ 8,077.5	$ 7,895.9	$ 6,668.2
Adjusted EBITDA									1,836.5	2,122.6	1,826.5
Total assets	15,809.0				16,733.0				15,809.0	16,733.0	11,363.1
Property, Plant and Equipment, Net	11,801.7				11,251.6				11,801.7	11,251.6	7,263.3
Additions to property, plant, equipment, and mine development									986.0	847.4	546.0
Federal Coal Lease Expenditures									276.5	42.4	0
Income (loss) from equity affiliates									(61.2)	(19.2)	(1.7)
Reconciliation Of Adjusted EBITDA To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA									1,836.5	2,122.6	1,826.5
Depreciation, depletion and amortization									(663.4)	(474.3)	(429.5)
Asset retirement obligation expenses									(67.0)	(52.6)	(45.9)
Asset impairment and mine closure costs	921.3	7.7							929.0	0	0
Amortization of basis difference									(4.6)	0	0
Interest expense									(405.6)	(238.6)	(222.0)
Interest income									24.5	18.9	9.6
Income tax provision									(262.3)	(363.2)	(313.7)
(Loss) Income from continuing operations, net of income taxes	(991.3)	122.9	214.5	183.0	221.5	291.2	306.5	193.6	(470.9)	1,012.8	825.0
Revenue from external customers by geographic region
Revenue percentage									100.00%	100.00%	100.00%
Western U.S. Mining [Member]
Operating segment results
Revenues									2,949.3	2,900.4	2,706.3
Adjusted EBITDA									832.8	766.0	816.7
Total assets	3,248.6				3,095.8				3,248.6	3,095.8	3,008.4
Property, Plant and Equipment, Net	2,886.8				2,785.1				2,886.8	2,785.1	2,672.8
Additions to property, plant, equipment, and mine development									121.4	186.1	143.3
Federal Coal Lease Expenditures									276.5	42.4
Income (loss) from equity affiliates									0	0	0
Reconciliation Of Adjusted EBITDA To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA									832.8	766.0	816.7
Asset impairment and mine closure costs									2.4
Midwestern U.S. Mining [Member]
Operating segment results
Revenues									1,403.7	1,402.6	1,248.9
Adjusted EBITDA									427.0	402.9	309.9
Total assets	507.7				590.6				507.7	590.6	528.5
Property, Plant and Equipment, Net	486.6				502.4				486.6	502.4	508.4
Additions to property, plant, equipment, and mine development									74.7	99.9	223.0
Federal Coal Lease Expenditures									0	0
Income (loss) from equity affiliates									0	0	0
Reconciliation Of Adjusted EBITDA To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA									427.0	402.9	309.9
Asset impairment and mine closure costs									45.0
Australian Mining [Member]
Operating segment results
Revenues									3,503.6	3,080.7	2,399.9
Adjusted EBITDA									938.9	1,194.3	977.4
Total assets	7,990.8				8,440.4				7,990.8	8,440.4	3,556.9
Property, Plant and Equipment, Net	6,595.5				6,371.3				6,595.5	6,371.3	2,559.4
Additions to property, plant, equipment, and mine development									743.4	490.8	138.7
Federal Coal Lease Expenditures									0	0
Income (loss) from equity affiliates									0	0	0
Reconciliation Of Adjusted EBITDA To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA									938.9	1,194.3	977.4
Asset impairment and mine closure costs									806.7
Trading and Brokerage [Member]
Operating segment results
Revenues									199.9	475.1	291.1
Adjusted EBITDA									119.7	197.0	77.2
Total assets	544.4				633.3				544.4	633.3	398.2
Property, Plant and Equipment, Net	2.1				1.7				2.1	1.7	1.2
Additions to property, plant, equipment, and mine development									0.1	0.9	0.9
Federal Coal Lease Expenditures									0	0
Income (loss) from equity affiliates									0	0	0
Reconciliation Of Adjusted EBITDA To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA									119.7	197.0	77.2
Corporate and Other [Member]
Operating segment results
Revenues									21.0	37.1	22.0
Adjusted EBITDA									(481.9)	(437.6)	(354.7)
Total assets	3,517.5				3,972.9				3,517.5	3,972.9	3,871.1
Property, Plant and Equipment, Net	1,830.7				1,591.1				1,830.7	1,591.1	1,521.5
Additions to property, plant, equipment, and mine development									46.4	69.7	40.1
Federal Coal Lease Expenditures									0	0
Income (loss) from equity affiliates									(61.2)	(19.2)	1.7
Reconciliation Of Adjusted EBITDA To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA									(481.9)	(437.6)	(354.7)
Asset impairment and mine closure costs									$ 74.9
U.S. [Member]
Revenue from external customers by geographic region
Revenue percentage									55.40%	61.00%	63.50%
Japan [Member]
Revenue from external customers by geographic region
Revenue percentage									12.30%	10.20%	9.10%
China
Revenue from external customers by geographic region
Revenue percentage									6.80%	2.80%	2.70%
South Korea [Member]
Revenue from external customers by geographic region
Revenue percentage									5.80%	5.50%	3.70%
India [Member]
Revenue from external customers by geographic region
Revenue percentage									4.10%	5.20%	4.90%
Other [Member]
Revenue from external customers by geographic region
Revenue percentage									15.60%	15.30%	16.10%