Financial Instruments (Details) $ in Millions		3 Months Ended	12 Months Ended
		Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jul. 26, 2016 USD ($)
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			$ 2,015.5
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Amount in bank guarantees, letters of credit and surety bonds related to road maintenance, performance guarantees and other operations			121.0
Amount in Letters of Credit Held as Collateral in Support of Surety Bonds			48.0
Amount in letters of credit issued as collateral to support surety bonds			72.6
Cash collateral in support of reclamation obligations or activities			233.2
Letters of credit in support of reclamation obligations or activities		$ 15.7	80.0	$ 126.6
Total consideration received by Company related to accounts receivable sold under securitization program			2,859.9
Cash up front from sale of receivables			1,541.7
Additional cash upon collection of underlying receivables			1,155.3
Non collected receivables			162.9
Reduction in accounts receivable as a result of securitization activity				168.5
Expense associated with securitization transactions			8.2	1.8	$ 1.5
Restricted Cash and Investments, Current			54.3	0.0
Maximum Capacity of Securitization Program			180.0	$ 275.0
Restricted Cash and Cash Equivalents			40.5
Restricted Cash and Investments, Noncurrent			71.4
DTA and PBGC [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Letters of credit outstanding			$ 37.0
Dominion Terminal Associates Partnership [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Ownership percentage of equity method investment			37.50%
DTA Lease Term			30
Pension Plans Agreement With PBG and TXU Europe [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Defined benefit pension plans requiring special contributions			2
Letter of credit maintained by the Company in favor of the PBGC			$ 37.0
Guarantee in place from TXU Europe Limited			110.0
Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			1,701.7
Coal Lease Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			94.0
Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			61.8
Other [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk	[1]		158.0
Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			381.5
Self bonding [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			1,094.2
Self bonding [Member] | Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			1,094.2
Self bonding [Member] | Coal Lease Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			0.0
Self bonding [Member] | Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			0.0
Self bonding [Member] | Other [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk	[1]		0.0
Self bonding [Member] | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			0.0
Surety Bond [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			448.2
Surety Bond [Member] | Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			319.6
Surety Bond [Member] | Coal Lease Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			94.0
Surety Bond [Member] | Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			19.1
Surety Bond [Member] | Other [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk	[1]		15.5
Surety Bond [Member] | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk	[2]		64.5
Bank Guarantees [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			79.2
Bank Guarantees [Member] | Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			54.7
Bank Guarantees [Member] | Coal Lease Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			0.0
Bank Guarantees [Member] | Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			0.0
Bank Guarantees [Member] | Other [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk	[1]		24.5
Bank Guarantees [Member] | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			83.8
Letters of credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			393.9
Letters of credit [Member] | Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			233.2
Letters of credit [Member] | Coal Lease Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			0.0
Letters of credit [Member] | Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			42.7
Letters of credit [Member] | Other [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk	[1]		118.0
Letters of credit [Member] | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off balance sheet risk			233.2
Parent Company [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Restricted Cash and Investments, Current			13.8
Wyoming [Member] | Self bonding [Member] | Reclamation Obligations [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Letters of credit in support of reclamation obligations or activities						$ 0.8
Indiana [Member] | Self bonding [Member] | Reclamation Obligations [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Letters of credit in support of reclamation obligations or activities						7.5
Illinois [Member] | Self bonding [Member] | Reclamation Obligations [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Letters of credit in support of reclamation obligations or activities						$ 3.2
Scenario, Actual [Member] | Dominion Terminal Associates Partnership [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Repayments of Lines of Credit		39.9
Line of Credit Assumed		$ 42.7
U.S Mining [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Surety Bonds Outstanding For Reclamation			1,413.8
Costs Incurred, Asset Retirement Obligation Incurred			471.1
Australian Mining [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Costs Incurred, Asset Retirement Obligation Incurred			287.7
Cash collateral in support of reclamation obligations or activities			$ 233.2

[1] Other includes the $37.0 million in letters of credit related to the PBGC, as described below, and an additional $121.0 million in bank guarantees, letters of credit and surety bonds related to road maintenance, performance guarantees and other operations.

[2] A total of $72.6 million of letters of credit issued as collateral to support surety bonds related to Patriot have been excluded from above as they no longer represent off-balance sheet obligations as discussed in Note 27. "Matters Related to the Bankruptcy of Patriot Coal Corporation".