Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash Flows From Operating Activities
Net cash provided (used in) by continuing operations		$ 1,599.8	$ 1,652.1	$ 1,104.5
Net cash used in discontinued operations		(84.7)	(18.9)	(17.4)
Net cash provided by (used in) operating activities		1,515.1	1,633.2	1,087.1
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		(986.0)	(847.4)	(546.0)
Changes in accrued expenses related to capital expenditures		104.7	51.2	0
Federal Coal Lease Expenditures		(276.5)	(42.4)	0
Investment in Prairie State Energy Campus		(10.7)	(36.2)	(76.0)
Proceeds from disposal of assets, net of notes receivable		147.9	40.1	19.1
Investments in equity affiliates and joint ventures		0	(39.7)	(18.8)
Purchases of debt and equity securities		(46.7)	(147.7)	(74.6)
Proceeds from sales of debt and equity securities		46.4	104.6	12.4
Purchases of short-term investments		(4.8)	(100.0)	0
Maturity of short-term investments		0	100.0	0
Acquisition of Macarthur Coal Limited, net of cash acquired		0	(2,756.7)	0
Contributions to joint ventures		(824.0)	(145.4)	0
Distributions from joint ventures		823.0	128.6	0
Advances to related parties		(148.0)	(371.3)	0
Repayment of loan from related parties		110.8	331.7	0
Other, net		(6.2)	(6.6)	(8.8)
Net cash used in continuing operations		(1,070.1)	(3,737.2)	(692.7)
Net cash (used in) provided by discontinued operations		(22.0)	(70.6)	(10.9)
Net cash used in investing activities		(1,092.1)	(3,807.8)	(703.6)
Cash Flows From Financing Activities
Repayments of long-term debt		(415.8)	(263.9)	(1,167.3)
Proceeds from long-term debt		0.8	4,101.4	1,150.0
Payment of debt issuance costs		(6.9)	(61.5)	(32.2)
Dividends paid		(91.9)	(92.1)	(79.4)
Common stock repurchase	(100.0)	(99.9)	0	0
Repurchase of employee common stock relinquished for tax withholding		(8.4)	(18.7)	(13.5)
Excess tax benefit related to share-based compensation		8.3	8.1	51.0
Acquisition of PEAMCoal Pty Ltd noncontrolling interests		0	(1,994.8)	0
Acqiusition of MCG Coal Holdings Pty Ltd noncontrolling interests		49.8	0	0
Other, net		0.3	0	14.3
Transactions with affiliates, net		0	0	0
Net cash (used in) provided by financing activities		(663.3)	1,678.5	(77.1)
Net change in cash and cash equivalents		(240.3)	(496.1)	306.4
Cash and cash equivalents at beginning of year		799.1	1,295.2	988.8
Cash and cash equivalents at end of year		558.8	799.1	1,295.2
Parent Company [Member]
Cash Flows From Operating Activities
Net cash provided (used in) by continuing operations		372.0	28.9	(126.4)
Net cash used in discontinued operations		(3.4)	4.4	(14.2)
Net cash provided by (used in) operating activities		368.6	33.3	(140.6)
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		0	0	0
Changes in accrued expenses related to capital expenditures		0	0
Federal Coal Lease Expenditures		0	0
Investment in Prairie State Energy Campus		0	0	0
Proceeds from disposal of assets, net of notes receivable		0	0	0
Investments in equity affiliates and joint ventures			0	0
Purchases of debt and equity securities		0	0	0
Proceeds from sales of debt and equity securities		0	0	0
Purchases of short-term investments		0	(75.0)
Maturity of short-term investments			75.0
Acquisition of Macarthur Coal Limited, net of cash acquired			0
Contributions to joint ventures		0	0
Distributions from joint ventures		0	0
Advances to related parties		0	0
Repayment of loan from related parties		0	0
Other, net		0	0	0
Net cash used in continuing operations		0	0	0
Net cash (used in) provided by discontinued operations		0	0	0
Net cash used in investing activities		0	0	0
Cash Flows From Financing Activities
Repayments of long-term debt		(379.0)	(243.1)	(1,146.8)
Proceeds from long-term debt		0	4,100.0	1,150.0
Payment of debt issuance costs		(6.9)	(61.5)	(32.2)
Dividends paid		(91.9)	(92.1)	(79.4)
Common stock repurchase		(99.9)
Repurchase of employee common stock relinquished for tax withholding		(8.4)	(18.7)	(13.5)
Excess tax benefit related to share-based compensation		8.3	8.1	51.0
Acquisition of PEAMCoal Pty Ltd noncontrolling interests			(11.1)
Acqiusition of MCG Coal Holdings Pty Ltd noncontrolling interests		0
Other, net		9.4	11.1	22.3
Transactions with affiliates, net		186.2	(4,346.6)	724.6
Net cash (used in) provided by financing activities		(382.2)	(653.9)	676.0
Net change in cash and cash equivalents		(13.6)	(620.6)	535.4
Cash and cash equivalents at beginning of year		283.2	903.8	368.4
Cash and cash equivalents at end of year		269.6	283.2	903.8
Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash provided (used in) by continuing operations		1,390.3	1,513.7	1,078.4
Net cash used in discontinued operations		(11.6)	(0.8)	9.8
Net cash provided by (used in) operating activities		1,378.7	1,512.9	1,088.2
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		(237.4)	(350.4)	(402.6)
Changes in accrued expenses related to capital expenditures		3.1	0
Federal Coal Lease Expenditures		(276.5)	(42.4)
Investment in Prairie State Energy Campus		(10.7)	(36.2)	(76.0)
Proceeds from disposal of assets, net of notes receivable		70.1	34.1	14.0
Investments in equity affiliates and joint ventures			(2.0)	(15.0)
Purchases of debt and equity securities		0	(103.6)	0
Proceeds from sales of debt and equity securities		0	47.0	0
Purchases of short-term investments		0	0
Maturity of short-term investments			0
Acquisition of Macarthur Coal Limited, net of cash acquired			0
Contributions to joint ventures		0	0
Distributions from joint ventures		0	0
Advances to related parties		0	0
Repayment of loan from related parties		0	0
Other, net		(6.2)	(6.5)	(8.7)
Net cash used in continuing operations		(457.6)	(460.0)	(488.3)
Net cash (used in) provided by discontinued operations		(1.1)	(8.3)	(1.8)
Net cash used in investing activities		(458.7)	(468.3)	(490.1)
Cash Flows From Financing Activities
Repayments of long-term debt		(0.4)	0	0
Proceeds from long-term debt		0	0	0
Payment of debt issuance costs		0	0	0
Dividends paid		0	0	0
Common stock repurchase		0
Repurchase of employee common stock relinquished for tax withholding		0	0	0
Excess tax benefit related to share-based compensation		0	0	0
Acquisition of PEAMCoal Pty Ltd noncontrolling interests			11.1
Acqiusition of MCG Coal Holdings Pty Ltd noncontrolling interests		0
Other, net		(7.5)	(12.0)	(5.9)
Transactions with affiliates, net		(912.6)	(1,048.1)	(587.2)
Net cash (used in) provided by financing activities		(920.5)	(1,049.0)	(593.1)
Net change in cash and cash equivalents		(0.5)	(4.4)	5.0
Cash and cash equivalents at beginning of year		0.8	5.2	0.2
Cash and cash equivalents at end of year		0.3	0.8	5.2
Non-Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash provided (used in) by continuing operations		(162.5)	109.5	152.5
Net cash used in discontinued operations		(69.7)	(22.5)	(13.0)
Net cash provided by (used in) operating activities		(232.2)	87.0	139.5
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		(748.6)	(497.0)	(143.4)
Changes in accrued expenses related to capital expenditures		101.6	51.2
Federal Coal Lease Expenditures		0	0
Investment in Prairie State Energy Campus		0	0	0
Proceeds from disposal of assets, net of notes receivable		77.8	6.0	5.1
Investments in equity affiliates and joint ventures			(37.7)	(3.8)
Purchases of debt and equity securities		(46.7)	(44.1)	(74.6)
Proceeds from sales of debt and equity securities		46.4	57.6	12.4
Purchases of short-term investments		(4.8)	(25.0)
Maturity of short-term investments			25.0
Acquisition of Macarthur Coal Limited, net of cash acquired			(2,756.7)
Contributions to joint ventures		(824.0)	(145.4)
Distributions from joint ventures		823.0	128.6
Advances to related parties		(148.0)	(371.3)
Repayment of loan from related parties		110.8	331.7
Other, net		0	(0.1)	(0.1)
Net cash used in continuing operations		(612.5)	(3,277.2)	(204.4)
Net cash (used in) provided by discontinued operations		(20.9)	(62.3)	(9.1)
Net cash used in investing activities		(633.4)	(3,339.5)	(213.5)
Cash Flows From Financing Activities
Repayments of long-term debt		(36.4)	(20.8)	(20.5)
Proceeds from long-term debt		0.8	1.4	0
Payment of debt issuance costs		0	0	0
Dividends paid		0	0	0
Common stock repurchase		0
Repurchase of employee common stock relinquished for tax withholding		0	0	0
Excess tax benefit related to share-based compensation		0	0	0
Acquisition of PEAMCoal Pty Ltd noncontrolling interests			(1,994.8)
Acqiusition of MCG Coal Holdings Pty Ltd noncontrolling interests		49.8
Other, net		(1.6)	0.9	(2.1)
Transactions with affiliates, net		726.4	5,394.7	(137.4)
Net cash (used in) provided by financing activities		639.4	3,381.4	(160.0)
Net change in cash and cash equivalents		(226.2)	128.9	(234.0)
Cash and cash equivalents at beginning of year		515.1	386.2	620.2
Cash and cash equivalents at end of year		$ 288.9	$ 515.1	$ 386.2