Consolidated Statements of Changes in Stockholders' Equity (Unaudited) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Dec. 31, 2011
Retained Earnings	$ 4,095.2	$ 4,095.2	$ 3,744.0
Accumulated Other Comprehensive Income (Loss)	23.2	23.2	(142.4)
Noncontrolling interests	32.4	32.4	30.7
Net income	41.6	427.7
Net Income (Loss) Attributable to Noncontrolling Interest	(1.3)	7.4
Comprehensive income:
Net unrealized losses on available-for-sale securities (net of $11.2 tax benefit)	(5.1)	(19.2)
Increase in fair value of cash flow hedges (net of $39.7 tax provision)	53.8	122.8
Postretirement plans and workers' compensation obligations (net of $24.2 tax provision)	13.9	41.6
Foreign currency translation adjustment	13.8	20.4
Dividends paid		(69.1)
Share-based compensation		34.9
Excess tax benefits related to share-based compensation		3.6
Stock options exercised		1.7
Employee stock purchases		7.1
Repurchase of employee common stock relinquished for tax withholding		(8.3)
Common stock repurchase		(99.9)
MCG Coal Holdings Pty Ltd. noncontrolling interests at conversion		53.4
Acquisition of MCG Coal Holdings Pty Ltd noncontrolling interests		(49.8)
Distributions to noncontrolling interests		(5.7)
Ending Balance	5,977.0	5,977.0	5,515.8
Additional Paid-in Capital	2,284.9	2,284.9	2,234.0
Treasury Stock	(461.5)	(461.5)	353.3
Common Stock
Comprehensive income:
Share-based compensation		0
Stock options exercised		0
Employee stock purchases		0
Repurchase of employee common stock relinquished for tax withholding		0
Common stock repurchase		0
Ending Balance	2.8	2.8	2.8
Additional Paid-in Capital
Comprehensive income:
Dividends paid		0
Share-based compensation		34.9
Excess tax benefits related to share-based compensation		3.6
Stock options exercised		1.7
Employee stock purchases		7.1
Repurchase of employee common stock relinquished for tax withholding		0
Acquisition of MCG Coal Holdings Pty Ltd noncontrolling interests		3.6
Ending Balance			2,234.0
Additional Paid-in Capital	2,284.9	2,284.9
Treasury Stock
Comprehensive income:
Repurchase of employee common stock relinquished for tax withholding		(8.3)
Common stock repurchase		(99.9)
Ending Balance			(353.3)
Treasury Stock	(461.5)	(461.5)
Retained Earnings
Retained Earnings	4,095.2	4,095.2
Net income		420.3
Comprehensive income:
Dividends paid		(69.1)
Ending Balance			3,744.0
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss)	23.2	23.2
Comprehensive income:
Net unrealized losses on available-for-sale securities (net of $11.2 tax benefit)		(19.2)
Increase in fair value of cash flow hedges (net of $39.7 tax provision)		122.8
Postretirement plans and workers' compensation obligations (net of $24.2 tax provision)		41.6
Foreign currency translation adjustment		20.4
Ending Balance			(142.4)
Noncontrolling Interests
Noncontrolling interests	32.4	32.4
Comprehensive income:
Net unrealized losses on available-for-sale securities (net of $11.2 tax benefit)		0
Increase in fair value of cash flow hedges (net of $39.7 tax provision)		0
Postretirement plans and workers' compensation obligations (net of $24.2 tax provision)		0
Foreign currency translation adjustment		0
MCG Coal Holdings Pty Ltd. noncontrolling interests at conversion		53.4
Acquisition of MCG Coal Holdings Pty Ltd noncontrolling interests		(53.4)
Distributions to noncontrolling interests		(5.7)
Ending Balance			$ 30.7