Consolidated Statements of Changes in Stockholders' Equity (USD $) In Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Noncontrolling Interests [Member]
Beginning Balance at Dec. 31, 2009	$ 3,755.9	$ 2.8	$ 2,067.7	$ (321.1)	$ 2,183.8	$ (183.5)	$ 6.2
Net (loss) income	802.2				774.0
Net Income (Loss) Attributable to Parent	774.0
Less: Net income (loss) attributable to noncontrolling interests	28.2						28.2
Net unrealized gains on cash flow hedges	127.5					127.5
Postretirement plan and worker's compensation obligations	(11.9)					(11.9)
Foreign currency translation adjustment	0
Dividends paid	(79.4)				(79.4)
Share-based compensation	41.1		41.1
Excess tax benefits related to share based compensation	51.0		51.0
Excess Tax Benefit from Share-based Compensation, Financing Activities	51.0
Stock options exercised	16.4	0	16.4
Employee stock purchases	5.8	0	5.8
Repurchase of employee common stock relinquished for tax withholding	(13.5)			(13.5)
Common stock repurchase	0
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	0
Noncontrolling Interest, Increase from Business Combination	0
Distributions to noncontrolling interests	(5.8)						(5.8)
Ending Balance at Dec. 31, 2010	4,689.3	2.8	2,182.0	(334.6)	2,878.4	(67.9)	28.6
Net (loss) income	946.3				957.7
Net Income (Loss) Attributable to Parent	957.7
Less: Net income (loss) attributable to noncontrolling interests	(11.4)						(11.4)
Net change in unrealized losses on available-for-sale securities, net of tax	(6.7)					(6.7)	0
Net unrealized gains on cash flow hedges	40.9					40.9	0
Postretirement plan and worker's compensation obligations	(108.7)					(108.7)	0
Foreign currency translation adjustment	0
Dividends paid	(92.1)				(92.1)
Share-based compensation	43.9		43.9
Excess tax benefits related to share based compensation	8.1		8.1
Excess Tax Benefit from Share-based Compensation, Financing Activities	8.1
Stock options exercised	4.8	0	4.8
Employee stock purchases	6.3	0	6.3
Repurchase of employee common stock relinquished for tax withholding	(18.7)			(18.7)
Common stock repurchase	0
Macarthur Coal Ltd noncontrolling interests at control date	2,011.9						2,011.9
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	0
Noncontrolling Interest, Increase from Business Combination	(1,994.8)		(11.1)				(1,983.7)
Distributions to noncontrolling interests	(15.9)						(15.9)
Contributions from noncontrolling interests	1.2						1.2
Ending Balance at Dec. 31, 2011	5,515.8	2.8	2,234.0	(353.3)	3,744.0	(142.4)	30.7
Net (loss) income	(575.1)
Net Income (Loss) Attributable to Parent	(585.7)
Less: Net income (loss) attributable to noncontrolling interests	10.6						10.6
Net change in unrealized losses on available-for-sale securities, net of tax	7.0					7.0	0
Net unrealized gains on cash flow hedges	51.8					51.8	0
Postretirement plan and worker's compensation obligations	75.5					75.5	0
Foreign currency translation adjustment	19.1					19.1	0
Dividends paid	91.9				(91.9)
Share-based compensation	45.4		45.4
Excess tax benefits related to share based compensation			8.3
Excess Tax Benefit from Share-based Compensation, Financing Activities	8.3
Stock options exercised	2.4	0	2.4
Employee stock purchases	7.0	0	7.0
Repurchase of employee common stock relinquished for tax withholding	(8.4)			(8.4)
Common stock repurchase	(99.9)			(99.9)
Macarthur Coal Ltd noncontrolling interests at control date	39.0						39.0
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	(49.8)		(10.8)				(39.0)
Noncontrolling Interest, Increase from Business Combination	0
Distributions to noncontrolling interests	(7.4)						(7.4)
Ending Balance at Dec. 31, 2012	$ 4,938.8	$ 2.8	$ 2,286.3	$ (461.6)	$ 3,066.4	$ 11.0	$ 33.9