Supplemental Guarantor/Non-Guarantor Financial Information (Details 6) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	$ 246.6	$ (438.9)
Cash Provided by (Used in) Operating Activities, Discontinued Operations	(8.2)	(0.1)
Net Cash Provided by (Used in) Operating Activities	238.4	(439.0)
Additions to property, plant, equipment and mine development	(32.8)	(13.3)
Changes in accrued expenses related to capital expenditures	(1.4)	(3.4)
Federal coal lease expenditures	(0.5)	(0.4)
Proceeds from disposal of assets, net of notes receivable	24.3	2.1
Contributions to joint ventures	(95.4)	(81.7)
Distributions from joint ventures	90.5	87.4
Increase (Decrease) in Due to Related Parties	(0.4)	(1.2)
Increase (Decrease) in Due from Related Parties	31.1	2.1
Other, net	(0.3)	(4.5)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations	15.1
Proceeds from long-term debt	1,000.0	947.0
Restricted Cash Related to Proceeds from Long-term Debt	(1,000.0)	0.0
Repayments of long-term debt	(2.1)	(6.2)
Payment of deferred financing costs	(55.4)	(2.8)
Distributions to noncontrolling interests	(0.1)	0.0
Other, net	(0.1)	(1.8)
Transactions With Affiliates Net	0.0	0.0
Net Cash Provided by (Used in) Financing Activities	(57.7)	936.2
Cash and Cash Equivalents, Period Increase (Decrease)	195.8	484.3
Cash and Cash Equivalents, at Carrying Value	1,068.1	$ 745.6	$ 872.3	$ 261.3
Debtor Subsidiaries [Member]
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	(49.7)
Cash Provided by (Used in) Operating Activities, Discontinued Operations	(6.0)
Net Cash Provided by (Used in) Operating Activities	(55.7)
Additions to property, plant, equipment and mine development	(25.4)
Changes in accrued expenses related to capital expenditures	(0.6)
Federal Coal Lease Expenditures	(0.5)
Proceeds from disposal of assets, net of notes receivable	24.3
Contributions to joint ventures	0.0
Distributions from joint ventures	0.0
Increase (Decrease) in Due to Related Parties	0.0
Increase (Decrease) in Due from Related Parties	0.0
Other, net	(0.5)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations	(1.5)
Proceeds from long-term debt	1,000.0
Restricted Cash Related to Proceeds from Long-term Debt	(1,000.0)
Repayments of long-term debt	(1.2)
Payment of deferred financing costs	(55.4)
Distributions to noncontrolling interests	0.0
Other, net	(0.1)
Transactions With Affiliates Net	(19.0)
Net Cash Provided by (Used in) Financing Activities	(75.7)
Cash and Cash Equivalents, Period Increase (Decrease)	(132.9)
Cash and Cash Equivalents, at Carrying Value	261.6		394.5
Non-Debtor Subsidiaries [Member]
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	296.3
Cash Provided by (Used in) Operating Activities, Discontinued Operations	(2.2)
Net Cash Provided by (Used in) Operating Activities	294.1
Additions to property, plant, equipment and mine development	(7.4)
Changes in accrued expenses related to capital expenditures	(2.0)
Federal Coal Lease Expenditures	0.0
Proceeds from disposal of assets, net of notes receivable	0.0
Contributions to joint ventures	(95.4)
Distributions from joint ventures	90.5
Increase (Decrease) in Due to Related Parties	(0.4)
Increase (Decrease) in Due from Related Parties	31.1
Other, net	0.2
Net Cash Provided by (Used in) Investing Activities, Continuing Operations	16.6
Proceeds from long-term debt	0.0
Restricted Cash Related to Proceeds from Long-term Debt	0.0
Repayments of long-term debt	(0.9)
Payment of deferred financing costs	0.0
Distributions to noncontrolling interests	(0.1)
Other, net	0.0
Transactions With Affiliates Net	19.0
Net Cash Provided by (Used in) Financing Activities	18.0
Cash and Cash Equivalents, Period Increase (Decrease)	328.7
Cash and Cash Equivalents, at Carrying Value	$ 806.5		$ 477.8