Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) (USD $)	3 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations		$ 441,000,000	$ 780,100,000	$ 1,599,800,000
Net cash used in discontinued operations		(104,400,000)	(57,700,000)	(84,700,000)
Net cash (used in) provided by operating activities		336,600,000	722,400,000	1,515,100,000
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		(194,400,000)	(328,400,000)	(986,000,000)
Changes in accrued expenses related to capital expenditures		(16,600,000)	(120,700,000)	104,700,000
Federal coal lease expenditures		276,700,000	276,800,000	276,500,000
Investment in Prairie State Energy Campus		0	0	(10,700,000)
Proceeds from disposal of assets, net of notes receivable		203,700,000	178,300,000	147,900,000
Purchases of debt and equity securities		(15,100,000)	(22,800,000)	(46,700,000)
Proceeds from sales and maturities of debt and equity securities		13,500,000	22,900,000	46,400,000
Purchases of short-term investments		0	0	(4,800,000)
Maturity of short-term investments		0	4,800,000	0
Contributions to joint ventures		(529,800,000)	(671,700,000)	(824,000,000)
Distributions from joint ventures		534,200,000	722,900,000	823,000,000
Advances to related parties		(33,700,000)	(42,100,000)	(148,000,000)
Repayment of loans from related parties		5,400,000	25,200,000	110,800,000
Other, net		(5,000,000)	(5,800,000)	(6,200,000)
Net cash used in continuing operations		(314,500,000)	(514,200,000)	(1,070,100,000)
Net cash used in discontinued operations		0	(1,500,000)	(22,000,000)
Net cash used in investing activities		(314,500,000)	(515,700,000)	(1,092,100,000)
Cash Flows From Financing Activities
Proceeds from long-term debt		1,100,000	1,188,000,000	800,000
Repayments of long-term debt		(21,000,000)	(1,390,200,000)	(415,800,000)
Payment of deferred financing costs		(10,100,000)	(22,800,000)	(6,900,000)
Dividends paid		(92,300,000)	(91,700,000)	(91,900,000)
Restricted cash for distributions to noncontrolling interests		(42,500,000)	0	0
Common stock repurchase	(99,900,000)	0	0	(99,900,000)
Excess tax benefit related to share-based compensation		0	0	8,300,000
Acquisition of MCG Coal Holdings Pty Ltd noncontrolling interests		0	0	49,800,000
Other, net		(3,300,000)	(4,800,000)	(8,100,000)
Transactions with affiliates, net		0	0	0
Net cash provided by (used in) financing activities		(168,100,000)	(321,500,000)	(663,300,000)
Net change in cash and cash equivalents		(146,000,000)	(114,800,000)	(240,300,000)
Cash and cash equivalents at beginning of year		444,000,000	558,800,000	799,100,000
Cash and cash equivalents at end of year		298,000,000	444,000,000	558,800,000
Parent Company [Member]
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations		(369,000,000)	(24,300,000)	372,000,000
Net cash used in discontinued operations		(73,300,000)	(20,400,000)	(3,400,000)
Net cash (used in) provided by operating activities		(442,300,000)	(44,700,000)	368,600,000
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	0
Federal coal lease expenditures		0	0	0
Investment in Prairie State Energy Campus				0
Proceeds from disposal of assets, net of notes receivable		0	0	0
Purchases of debt and equity securities		0	0	0
Proceeds from sales and maturities of debt and equity securities		0	0	0
Purchases of short-term investments				0
Maturity of short-term investments			0
Contributions to joint ventures		0	0	0
Distributions from joint ventures		0	0	0
Advances to related parties		0	0	0
Repayment of loans from related parties		0	0	0
Other, net		0	0	0
Net cash used in continuing operations			0	0
Net cash used in discontinued operations			0	0
Net cash used in investing activities		0	0	0
Cash Flows From Financing Activities
Proceeds from long-term debt		0	1,188,000,000	0
Repayments of long-term debt		(12,000,000)	(1,334,200,000)	(379,000,000)
Payment of deferred financing costs		(10,100,000)	(22,800,000)	(6,900,000)
Dividends paid		(92,300,000)	(91,700,000)	(91,900,000)
Restricted cash for distributions to noncontrolling interests		0
Common stock repurchase				(99,900,000)
Excess tax benefit related to share-based compensation				8,300,000
Acquisition of MCG Coal Holdings Pty Ltd noncontrolling interests				0
Other, net		3,100,000	4,200,000	1,000,000
Transactions with affiliates, net		441,600,000	332,300,000	186,200,000
Net cash provided by (used in) financing activities		330,300,000	75,800,000	(382,200,000)
Net change in cash and cash equivalents		(112,000,000)	31,100,000	(13,600,000)
Cash and cash equivalents at beginning of year		300,700,000	269,600,000	283,200,000
Cash and cash equivalents at end of year		188,700,000	300,700,000	269,600,000
Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations		776,100,000	778,700,000	1,390,300,000
Net cash used in discontinued operations		(4,600,000)	(7,600,000)	(11,600,000)
Net cash (used in) provided by operating activities		771,500,000	771,100,000	1,378,700,000
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		(108,500,000)	(95,900,000)	(237,400,000)
Changes in accrued expenses related to capital expenditures		3,400,000	(1,200,000)	3,100,000
Federal coal lease expenditures		(276,700,000)	(276,800,000)	(276,500,000)
Investment in Prairie State Energy Campus				(10,700,000)
Proceeds from disposal of assets, net of notes receivable		105,900,000	93,000,000	70,100,000
Purchases of debt and equity securities		0	0	0
Proceeds from sales and maturities of debt and equity securities		0	0	0
Purchases of short-term investments				0
Maturity of short-term investments			0
Contributions to joint ventures		0	0	0
Distributions from joint ventures		0	0	0
Advances to related parties		0	0	0
Repayment of loans from related parties		0	0	0
Other, net		(4,400,000)	(5,700,000)	(6,200,000)
Net cash used in continuing operations			(286,600,000)	(457,600,000)
Net cash used in discontinued operations			0	(1,100,000)
Net cash used in investing activities		(280,300,000)	(286,600,000)	(458,700,000)
Cash Flows From Financing Activities
Proceeds from long-term debt		0	0	0
Repayments of long-term debt		(100,000)	(200,000)	(400,000)
Payment of deferred financing costs		0	0	0
Dividends paid		0	0	0
Restricted cash for distributions to noncontrolling interests		0
Common stock repurchase				0
Excess tax benefit related to share-based compensation				0
Acquisition of MCG Coal Holdings Pty Ltd noncontrolling interests				0
Other, net		(1,700,000)	(1,600,000)	0
Transactions with affiliates, net		(488,500,000)	(482,700,000)	(920,100,000)
Net cash provided by (used in) financing activities		(490,300,000)	(484,500,000)	(920,500,000)
Net change in cash and cash equivalents		900,000	0	(500,000)
Cash and cash equivalents at beginning of year		300,000	300,000	800,000
Cash and cash equivalents at end of year		1,200,000	300,000	300,000
Non-Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations		33,900,000	25,700,000	(162,500,000)
Net cash used in discontinued operations		(26,500,000)	(29,700,000)	(69,700,000)
Net cash (used in) provided by operating activities		7,400,000	(4,000,000)	(232,200,000)
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		(85,900,000)	(232,500,000)	(748,600,000)
Changes in accrued expenses related to capital expenditures		(20,000,000)	(119,500,000)	101,600,000
Federal coal lease expenditures		0	0	0
Investment in Prairie State Energy Campus				0
Proceeds from disposal of assets, net of notes receivable		97,800,000	85,300,000	77,800,000
Purchases of debt and equity securities		(15,100,000)	(22,800,000)	(46,700,000)
Proceeds from sales and maturities of debt and equity securities		13,500,000	22,900,000	46,400,000
Purchases of short-term investments				(4,800,000)
Maturity of short-term investments			4,800,000
Contributions to joint ventures		(529,800,000)	(671,700,000)	(824,000,000)
Distributions from joint ventures		534,200,000	722,900,000	823,000,000
Advances to related parties		(33,700,000)	(42,100,000)	(148,000,000)
Repayment of loans from related parties		5,400,000	25,200,000	110,800,000
Other, net		(600,000)	(100,000)	0
Net cash used in continuing operations			(227,600,000)	(612,500,000)
Net cash used in discontinued operations			(1,500,000)	(20,900,000)
Net cash used in investing activities		(34,200,000)	(229,100,000)	(633,400,000)
Cash Flows From Financing Activities
Proceeds from long-term debt		1,100,000	0	800,000
Repayments of long-term debt		(8,900,000)	(55,800,000)	(36,400,000)
Payment of deferred financing costs		0	0	0
Dividends paid		0	0	0
Restricted cash for distributions to noncontrolling interests		(42,500,000)
Common stock repurchase
Excess tax benefit related to share-based compensation				0
Acquisition of MCG Coal Holdings Pty Ltd noncontrolling interests				(49,800,000)
Other, net		(4,700,000)	(7,400,000)	(9,100,000)
Transactions with affiliates, net		46,900,000	150,400,000	733,900,000
Net cash provided by (used in) financing activities		(8,100,000)	87,200,000	639,400,000
Net change in cash and cash equivalents		(34,900,000)	(145,900,000)	(226,200,000)
Cash and cash equivalents at beginning of year		143,000,000	288,900,000	515,100,000
Cash and cash equivalents at end of year		$ 108,100,000	$ 143,000,000	$ 288,900,000