Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) (Details) $ in Millions	3 Months Ended			12 Months Ended	48 Months Ended
	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Credit support to Patriot net letters of credit	$ 20.9
Letters of credit in support of reclamation obligations or activities	15.7
Potential exposure from Patriot bankruptcy	150.0
Black Lung Liability, Outstanding	$ 124.1
Number of VEBA payments from Patriot settlement	4
Payment to the VEBA based on the construct of the negotiated settlement				$ 75.0
Gain on VEBA settlement	$ 0.0		$ 68.1
Withdrawal liability related to pension plan	767.0
Loss from discontinued operations, net of income taxes	(4.1)		$ (3.4)
Accounts payable and accrued expenses	50.6	$ 55.9
Liabilities subject to compromise	8,416.7	8,440.2
Veba Payment [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Payment to the VEBA based on the construct of the negotiated settlement				7.5
Scenario, Forecast [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Funding of the newly established VEBA					$ 310.0
United Mine Workers of America [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Loss from discontinued operations, net of income taxes	$ 56.0	$ 54.3
Plan Effective Date [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Payment to the VEBA based on the construct of the negotiated settlement				5.0
90 Days after Plan Effective Date [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Payment to the VEBA based on the construct of the negotiated settlement				10.0
One year after previous payment date [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Payment to the VEBA based on the construct of the negotiated settlement				15.0
Remaining 3 years [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Payment to the VEBA based on the construct of the negotiated settlement				$ 15.0