Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) - USD ($) $ in Millions	6 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Apr. 12, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	$ 254.9	$ (257.9)	$ (62.0)
Cash Provided by (Used in) Operating Activities, Discontinued Operations	(18.2)	(18.9)	(28.6)
Net Cash Provided by (Used in) Operating Activities	236.7	(276.8)	(90.6)
Additions to property, plant, equipment and mine development	(36.7)	(56.6)	(76.9)
Changes in accrued expenses related to capital expenditures	(1.5)	(5.5)	(14.5)
Federal coal lease expenditures		(249.0)	(89.8)
Proceeds from disposal of assets, net of notes receivable	132.5	134.7	39.6
Payments to Acquire Marketable Securities		0.0	22.6
Proceeds from Sale and Maturity of Marketable Securities		0.0	33.6
Contributions to joint ventures	(140.5)	(241.7)	(349.8)
Distributions from joint ventures	139.7	236.7	339.8
Increase (Decrease) in Due to Related Parties	(22.7)	(23.3)	(3.6)
Increase (Decrease) in Due from Related Parties	11.9	13.2	0.0
Other, net	(2.9)	(8.2)	(2.1)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations	(168.7)
Net Cash Provided by (Used in) Investing Activities		(199.7)	(146.3)
Proceeds from long-term debt	482.8	1,429.8	975.7
Repayments of long-term debt	(4.1)	(11.2)	(666.0)
Payment of deferred financing costs	(27.0)	(29.8)	(28.7)
Payments of Ordinary Dividends, Common Stock		0.0	1.4
Payments For Repurchase Of Common Stock Relinquished For Tax Withholding		0.1
Distributions to noncontrolling interests	(3.9)	(3.9)	(2.1)
Other, net	(0.1)	(1.9)	(4.3)
Transactions With Affiliates Net	0.0	0.0	0.0
Net Cash Provided by (Used in) Financing Activities	447.7	1,383.0	273.2
Cash and Cash Equivalents, Period Increase (Decrease)	515.7	906.5	36.3
Cash and Cash Equivalents, at Carrying Value	1,167.8	1,167.8	334.3	$ 652.1	$ 261.3	$ 298.0
Parent Company [Member]
Net Cash Provided by (Used in) Operating Activities, Continuing Operations		(249.9)	(649.0)
Cash Provided by (Used in) Operating Activities, Discontinued Operations		(5.1)	(23.2)
Net Cash Provided by (Used in) Operating Activities		(255.0)	(672.2)
Additions to property, plant, equipment and mine development		0.0	0.0
Changes in accrued expenses related to capital expenditures		0.0	0.0
Federal coal lease expenditures		0.0	0.0
Proceeds from disposal of assets, net of notes receivable		0.0	0.0
Payments to Acquire Marketable Securities			0.0
Proceeds from Sale and Maturity of Marketable Securities			0.0
Contributions to joint ventures		0.0	0.0
Distributions from joint ventures		0.0	0.0
Increase (Decrease) in Due to Related Parties		0.0	0.0
Other, net		0.0	0.0
Net Cash Provided by (Used in) Investing Activities		0.0	0.0
Proceeds from long-term debt		1,422.0	975.7
Repayments of long-term debt		(3.0)	(659.0)
Payment of deferred financing costs		(25.6)	(28.7)
Payments of Ordinary Dividends, Common Stock			1.4
Distributions to noncontrolling interests		0.0	0.0
Other, net		0.0	1.4
Transactions With Affiliates Net		(458.1)	339.7
Net Cash Provided by (Used in) Financing Activities		935.3	627.7
Cash and Cash Equivalents, Period Increase (Decrease)		680.3	(44.5)
Cash and Cash Equivalents, at Carrying Value	687.5	687.5	144.2		7.2	188.7
Guarantor Subsidiaries [Member]
Net Cash Provided by (Used in) Operating Activities, Continuing Operations		89.3	437.0
Cash Provided by (Used in) Operating Activities, Discontinued Operations		(13.1)	(2.8)
Net Cash Provided by (Used in) Operating Activities		76.2	434.2
Additions to property, plant, equipment and mine development		(26.3)	(44.8)
Changes in accrued expenses related to capital expenditures		(1.1)	(3.4)
Federal coal lease expenditures		(249.0)	(89.8)
Proceeds from disposal of assets, net of notes receivable		68.4	7.6
Payments to Acquire Marketable Securities			0.0
Proceeds from Sale and Maturity of Marketable Securities			0.0
Contributions to joint ventures		0.0	0.0
Distributions from joint ventures		0.0	0.0
Increase (Decrease) in Due to Related Parties		0.0	0.0
Other, net		(4.6)	(1.8)
Net Cash Provided by (Used in) Investing Activities		(212.6)	(132.2)
Proceeds from long-term debt		0.0	0.0
Repayments of long-term debt		(3.3)	(0.6)
Payment of deferred financing costs		0.0	0.0
Payments of Ordinary Dividends, Common Stock			0.0
Distributions to noncontrolling interests		0.0	0.0
Other, net		(1.9)	(1.6)
Transactions With Affiliates Net		140.0	(299.2)
Net Cash Provided by (Used in) Financing Activities		134.8	(301.4)
Cash and Cash Equivalents, Period Increase (Decrease)		(1.6)	0.6
Cash and Cash Equivalents, at Carrying Value	3.1	3.1	4.2		4.7	3.6
Non-Guarantor Subsidiaries [Member]
Net Cash Provided by (Used in) Operating Activities, Continuing Operations		(97.3)	150.0
Cash Provided by (Used in) Operating Activities, Discontinued Operations		(0.7)	(2.6)
Net Cash Provided by (Used in) Operating Activities		(98.0)	147.4
Additions to property, plant, equipment and mine development		(30.3)	(32.1)
Changes in accrued expenses related to capital expenditures		(4.4)	(11.1)
Federal coal lease expenditures		0.0	0.0
Proceeds from disposal of assets, net of notes receivable		66.3	32.0
Payments to Acquire Marketable Securities			22.6
Proceeds from Sale and Maturity of Marketable Securities			33.6
Contributions to joint ventures		(241.7)	(349.8)
Distributions from joint ventures		236.7	339.8
Increase (Decrease) in Due to Related Parties		(23.3)	(3.6)
Increase (Decrease) in Due from Related Parties		13.2
Other, net		(3.6)	(0.3)
Net Cash Provided by (Used in) Investing Activities		12.9	(14.1)
Proceeds from long-term debt		7.8	0.0
Repayments of long-term debt		(4.9)	(6.4)
Payment of deferred financing costs		(4.2)	0.0
Payments of Ordinary Dividends, Common Stock			0.0
Distributions to noncontrolling interests		(3.9)	(2.1)
Other, net		0.0	(4.1)
Transactions With Affiliates Net		318.1	(40.5)
Net Cash Provided by (Used in) Financing Activities		312.9	(53.1)
Cash and Cash Equivalents, Period Increase (Decrease)		227.8	80.2
Cash and Cash Equivalents, at Carrying Value	$ 477.2	$ 477.2	$ 185.9		$ 249.4	$ 105.7