Consolidated Statements of Changes in Stockholders' Equity (Unaudited) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Net income	$ 178.3
Net Income (Loss) Attributable to Noncontrolling Interest	5.6
Comprehensive income:
Net unrealized losses on available-for-sale securities (net of $6.7 tax benefit)	(11.5)
Increase in fair value of cash flow hedges (net of $31.4 tax provision)	63.6
Postretirement plans and workers compensation obligations (net of $8.1 tax provision)	13.9
Foreign currency translation adjustment	12.7
Dividends paid	(23.2)
Share-based compensation	13.0
Excess tax benefits related to share-based compensation	3.5
Stock options exercised	1.4
Employee stock purchases	3.3
Repurchase of employee common stock relinquished for tax withholding	(7.9)
MCG Coal Holdings Pty Ltd. noncontrolling interests at conversion	53.4
Distributions to noncontrolling interests	(3.5)
Ending Balance	5,812.8	5,515.8
Common Stock
Net income	0
Comprehensive income:
Net unrealized losses on available-for-sale securities (net of $6.7 tax benefit)	0
Increase in fair value of cash flow hedges (net of $31.4 tax provision)	0
Postretirement plans and workers compensation obligations (net of $8.1 tax provision)	0
Foreign currency translation adjustment	0
Dividends paid	0
Share-based compensation	0
Excess tax benefits related to share-based compensation	0
Stock options exercised	0
Employee stock purchases	0
Repurchase of employee common stock relinquished for tax withholding	0
MCG Coal Holdings Pty Ltd. noncontrolling interests at conversion	0
Distributions to noncontrolling interests	0
Ending Balance	2.8	2.8
Additional Paid-in Capital
Net income	0
Comprehensive income:
Net unrealized losses on available-for-sale securities (net of $6.7 tax benefit)	0
Increase in fair value of cash flow hedges (net of $31.4 tax provision)	0
Postretirement plans and workers compensation obligations (net of $8.1 tax provision)	0
Foreign currency translation adjustment	0
Dividends paid	0
Share-based compensation	13.0
Excess tax benefits related to share-based compensation	3.5
Stock options exercised	1.4
Employee stock purchases	3.3
Repurchase of employee common stock relinquished for tax withholding	0
MCG Coal Holdings Pty Ltd. noncontrolling interests at conversion	0
Distributions to noncontrolling interests	0
Ending Balance	2,255.2	2,234.0
Treasury Stock
Net income	0
Comprehensive income:
Net unrealized losses on available-for-sale securities (net of $6.7 tax benefit)	0
Increase in fair value of cash flow hedges (net of $31.4 tax provision)	0
Postretirement plans and workers compensation obligations (net of $8.1 tax provision)	0
Foreign currency translation adjustment	0
Dividends paid	0
Share-based compensation	0
Excess tax benefits related to share-based compensation	0
Stock options exercised	0
Employee stock purchases	0
Repurchase of employee common stock relinquished for tax withholding	(7.9)
MCG Coal Holdings Pty Ltd. noncontrolling interests at conversion	0
Distributions to noncontrolling interests	0
Ending Balance	(361.2)	(353.3)
Retained Earnings
Net income	172.7
Comprehensive income:
Net unrealized losses on available-for-sale securities (net of $6.7 tax benefit)	0
Increase in fair value of cash flow hedges (net of $31.4 tax provision)	0
Postretirement plans and workers compensation obligations (net of $8.1 tax provision)	0
Foreign currency translation adjustment	0
Dividends paid	(23.2)
Share-based compensation	0
Excess tax benefits related to share-based compensation	0
Stock options exercised	0
Employee stock purchases	0
Repurchase of employee common stock relinquished for tax withholding	0
MCG Coal Holdings Pty Ltd. noncontrolling interests at conversion	0
Distributions to noncontrolling interests	0
Ending Balance	3,893.5	3,744.0
Accumulated Other Comprehensive Income (Loss)
Net income	0
Comprehensive income:
Net unrealized losses on available-for-sale securities (net of $6.7 tax benefit)	(11.5)
Increase in fair value of cash flow hedges (net of $31.4 tax provision)	63.6
Postretirement plans and workers compensation obligations (net of $8.1 tax provision)	(13.9)
Foreign currency translation adjustment	12.7
Dividends paid	0
Share-based compensation	0
Excess tax benefits related to share-based compensation	0
Stock options exercised	0
Employee stock purchases	0
Repurchase of employee common stock relinquished for tax withholding	0
MCG Coal Holdings Pty Ltd. noncontrolling interests at conversion	0
Distributions to noncontrolling interests	0
Ending Balance	(63.7)	(142.4)
Noncontrolling Interests
Net Income (Loss) Attributable to Noncontrolling Interest	5.6
Comprehensive income:
Net unrealized losses on available-for-sale securities (net of $6.7 tax benefit)	0
Increase in fair value of cash flow hedges (net of $31.4 tax provision)	0
Postretirement plans and workers compensation obligations (net of $8.1 tax provision)	0
Foreign currency translation adjustment	0
Dividends paid	0
Share-based compensation	0
Excess tax benefits related to share-based compensation	0
Stock options exercised	0
Employee stock purchases	0
Repurchase of employee common stock relinquished for tax withholding	0
MCG Coal Holdings Pty Ltd. noncontrolling interests at conversion	53.4
Distributions to noncontrolling interests	(3.5)
Ending Balance	$ 86.2	$ 30.7