Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) - 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
Net Cash Provided by (Used in) Investing Activities	15.1	(12.9)
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)
Payments For Repurchase Of Common Stock Relinquished For Tax Withholding	0.1
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
Parent Company [Member]
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	167.9	(331.7)
Cash Provided by (Used in) Operating Activities, Discontinued Operations	(6.0)	(1.4)
Net Cash Provided by (Used in) Operating Activities	161.9	(333.1)
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
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
Increase (Decrease) in Due from Related Parties		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	1,000.0	947.0
Restricted Cash Related to Proceeds from Long-term Debt	(1,000.0)
Repayments of long-term debt	0.0	(3.0)
Payment of deferred financing costs	(29.1)	(2.8)
Distributions to noncontrolling interests	0.0
Other, net	(0.1)	(0.1)
Transactions With Affiliates Net	(177.9)	21.7
Net Cash Provided by (Used in) Financing Activities	(207.1)	962.8
Cash and Cash Equivalents, Period Increase (Decrease)	(45.2)	629.7
Cash and Cash Equivalents, at Carrying Value	221.4	636.9	266.6	7.2
Guarantor Subsidiaries [Member]
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	(288.2)	(21.3)
Cash Provided by (Used in) Operating Activities, Discontinued Operations	(0.7)	1.9
Net Cash Provided by (Used in) Operating Activities	(288.9)	(19.4)
Additions to property, plant, equipment and mine development	(22.2)	(4.3)
Changes in accrued expenses related to capital expenditures	(0.4)	(1.6)
Federal coal lease expenditures	(0.5)	(0.4)
Proceeds from disposal of assets, net of notes receivable	24.3	2.0
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	(1.2)
Increase (Decrease) in Due from Related Parties		0.0
Other, net	(0.2)	(2.5)
Net Cash Provided by (Used in) Investing Activities	1.0	(8.0)
Proceeds from long-term debt	0.0	0.0
Restricted Cash Related to Proceeds from Long-term Debt	0.0
Repayments of long-term debt	(1.2)	(1.1)
Payment of deferred financing costs	0.0	0.0
Distributions to noncontrolling interests	0.0
Other, net	0.0	(1.6)
Transactions With Affiliates Net	191.5	33.8
Net Cash Provided by (Used in) Financing Activities	190.3	31.1
Cash and Cash Equivalents, Period Increase (Decrease)	(97.6)	3.7
Cash and Cash Equivalents, at Carrying Value	9.4	8.4	107.0	4.7
Non-Guarantor Subsidiaries [Member]
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	366.9	(85.9)
Cash Provided by (Used in) Operating Activities, Discontinued Operations	(1.5)	(0.6)
Net Cash Provided by (Used in) Operating Activities	365.4	(86.5)
Additions to property, plant, equipment and mine development	(10.6)	(9.0)
Changes in accrued expenses related to capital expenditures	(1.0)	(1.8)
Federal coal lease expenditures	0.0	0.0
Proceeds from disposal of assets, net of notes receivable	0.0	0.1
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)	0.0
Increase (Decrease) in Due from Related Parties	31.1	2.1
Other, net	(0.1)	(2.0)
Net Cash Provided by (Used in) Investing Activities	14.1	(4.9)
Proceeds from long-term debt	0.0	0.0
Restricted Cash Related to Proceeds from Long-term Debt	0.0
Repayments of long-term debt	(0.9)	(2.1)
Payment of deferred financing costs	(26.3)	0.0
Distributions to noncontrolling interests	(0.1)
Other, net	0.0	(0.1)
Transactions With Affiliates Net	(13.6)	(55.5)
Net Cash Provided by (Used in) Financing Activities	(40.9)	(57.7)
Cash and Cash Equivalents, Period Increase (Decrease)	338.6	(149.1)
Cash and Cash Equivalents, at Carrying Value	$ 837.3	$ 100.3	$ 498.7	$ 249.4