Supplemental Guarantor/Non-Guarantor Financial 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	Dec. 31, 2012 Parent Company [Member]	Dec. 31, 2011 Parent Company [Member]	Dec. 31, 2010 Parent Company [Member]	Dec. 31, 2012 Guarantor Subsidiaries [Member]	Dec. 31, 2011 Guarantor Subsidiaries [Member]	Dec. 31, 2010 Guarantor Subsidiaries [Member]	Dec. 31, 2012 Non-Guarantor Subsidiaries [Member]	Dec. 31, 2011 Non-Guarantor Subsidiaries [Member]	Dec. 31, 2010 Non-Guarantor Subsidiaries [Member]	Dec. 31, 2012 Eliminations [Member]	Dec. 31, 2011 Eliminations [Member]	Dec. 31, 2010 Eliminations [Member]	Dec. 31, 2012 Senior Notes One Member	Apr. 15, 2011 Senior Notes One Member	Dec. 31, 2012 Senior Notes Five [Member]
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Interest Rate, Stated Percentage																								5.875%	5.875%	6.875%
Unaudited Supplemental Condensed Consolidated Statements of Operations
Total 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	$ 0	$ 0	$ 0	$ 4,571.3	$ 4,679.9	$ 3,524.4	$ 3,729.2	$ 3,509.4	$ 3,916.9	$ (223.0)	$ (293.4)	$ (773.1)
Costs and expenses
Operating costs and expenses									5,932.7	5,477.6	4,637.7	(395.2)	(386.3)	(145.1)	3,344.1	3,526.2	2,413.1	3,206.8	2,631.1	3,142.8	(223.0)	(293.4)	(773.1)
Depreciation, depletion and amortization									663.4	474.3	429.5	0	0	0	315.6	289.5	286.5	347.8	184.8	143.0	0	0	0
Asset retirement obligation expenses									67.0	52.6	45.9	0	0	0	42.9	35.7	30.6	24.1	16.9	15.3	0	0	0
Selling and administrative expenses									268.8	268.2	232.2	38.7	34.0	31.6	199.7	215.0	194.3	30.4	19.2	6.3	0	0	0
Acquisition costs related to Macarthur Coal Limited					76.1	9.1			0	85.2	0		32.8			31.2			21.2			0
Other operating (income) loss:
Net (gain) loss on disposal or exchange of assets									(17.1)	(76.9)	(29.9)	0	0	0	(14.7)	(73.2)	(34.4)	(2.4)	(3.7)	4.5	0	0	0
Asset impairment and mine closure costs	921.3	7.7							929.0	0	0	35.5			86.8			806.7			0
(Income) loss from equity affiliates									61.2	19.2	1.7	720.0	(881.5)	(838.4)	12.5	8.6	7.1	48.7	10.6	6.0	(720.0)	881.5	827.0
Interest expense									405.6	238.6	222.0	413.9	243.1	219.7	13.9	47.3	52.7	475.6	67.2	17.9	(497.8)	(119.0)	(68.3)
Interest income									(24.5)	(18.9)	(9.6)	(309.8)	(50.9)	(18.8)	(149.1)	(31.1)	(21.8)	(63.4)	(55.9)	(37.3)	497.8	119.0	68.3
Unrealized (gain) loss on derivatives									0	0		0	0		(35.3)	(1.3)		35.3	1.3		0	0
(Loss) income from continuing operations before income taxes									(208.6)	1,376.0	1,138.7	(503.1)	1,008.8	751.0	754.9	632.0	596.3	(1,180.4)	616.7	618.4	720.0	(881.5)	(827.0)
Income tax provision									262.3	363.2	313.7	80.2	46.5	(24.2)	105.8	172.0	181.6	76.3	144.7	156.3	0	0	0
(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	(583.3)	962.3	775.2	649.1	460.0	414.7	(1,256.7)	472.0	462.1	720.0	(881.5)	(827.0)
Income (loss) from discontinued operations, net of income taxes									(104.2)	(66.5)	(22.8)	(2.4)	(4.6)	(1.2)	(82.2)	(5.4)	(21.6)	(19.6)	(56.5)	0	0	0	0
Net (loss) income	(1,002.8)	41.6	207.8	178.3	194.0	281.4	292.2	178.7	(575.1)	946.3	802.2	(585.7)	957.7	774.0	566.9	454.6	393.1	(1,276.3)	415.5	462.1	720.0	(881.5)	(827.0)
Less: Net loss attributable to noncontrolling interests									10.6	(11.4)	28.2	0	0	0	0	0	0	10.6	(11.4)	28.2	0	0	0
Net (loss) income attributable to common stockholders	$ (1,006.0)	$ 42.9	$ 204.7	$ 172.7	$ 222.4	$ 274.0	$ 284.8	$ 176.5	$ (585.7)	$ 957.7	$ 774.0	$ (585.7)	$ 957.7	$ 774.0	$ 566.9	$ 454.6	$ 393.1	$ (1,286.9)	$ 426.9	$ 433.9	$ 720.0	$ (881.5)	$ (827.0)