Segment Information (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2017		Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Gain on VEBA settlement	$ 0.0		$ 68.1
Total Assets	12,836.5			$ 11,777.7
Reportable segment results
Revenues	1,326.2		1,027.2
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Depreciation, depletion and amortization	(119.9)		(111.8)
Asset impairment			14.3
Asset retirement obligation expenses	(14.6)		(13.1)
Selling and Administrative expenses related to debt restructuring	0.0
Asset Impairment Charges	30.5		17.2	$ 247.9	$ 1,277.8	$ 154.4
Interest expense	(32.9)		(126.2)
Unrealized gains on non-coal trading derivatives contracts	0.0		(25.0)
Interest income	2.7		1.4
Reorganization Items, net	(41.4)		0.0
Income tax (benefit) provision	(2.2)		59.8
Loss from continuing operations, net of income taxes	124.3		(167.7)
Segment Reporting Information Adjusted Earning Before Interest Taxes Depreciation And Amortization	357.9		30.1
Net gain on disposal of assets	22.8		1.8
Powder River Basin Mining [Member]
Reportable segment results
Revenues	394.3		336.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Segment Reporting Information Adjusted Earning Before Interest Taxes Depreciation And Amortization	91.7		73.8
Midwestern U.S. Mining [Member]
Reportable segment results
Revenues	193.2		199.6
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Segment Reporting Information Adjusted Earning Before Interest Taxes Depreciation And Amortization	50.0		60.6
Western U.S. Mining [Member]
Reportable segment results
Revenues	149.7		112.5
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Segment Reporting Information Adjusted Earning Before Interest Taxes Depreciation And Amortization	50.0		20.1
Australian Metallurgical Mining [Member]
Reportable segment results
Revenues	328.9		205.1
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Asset Impairment Charges			17.2
Segment Reporting Information Adjusted Earning Before Interest Taxes Depreciation And Amortization	109.6		(37.3)
Australian Thermal Mining [Member]
Reportable segment results
Revenues	224.8		176.7
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Segment Reporting Information Adjusted Earning Before Interest Taxes Depreciation And Amortization	75.6		42.9
Trading and Brokerage [Member]
Reportable segment results
Revenues	31.6		(8.8)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Segment Reporting Information Adjusted Earning Before Interest Taxes Depreciation And Amortization	25.4		(18.8)
Corporate and Other [Member]
Reportable segment results
Revenues	3.7		6.1
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Segment Reporting Information Adjusted Earning Before Interest Taxes Depreciation And Amortization	(44.4)	[1]	(111.2)
Middlemount Mine [Member]
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Change in deferred tax asset valuation allowance related to equity affiliates	(5.2)		$ 1.4
DTA and PBGC [Member]
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Net gain on disposal of assets	$ (19.7)

[1] (1) Includes a gain of $68.1 million related to the 2016 Settlement Agreement described in Note 20 "Matters related to the Bankruptcy of Patriot Coal Corporation"