Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) (Details)	3 Months Ended				9 Months Ended		12 Months Ended				48 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2017 USD ($)	Jan. 31, 2014 USD ($)	Oct. 31, 2007 USD ($)
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
CreditSupportToPatriotNetLettersOfCredit	$ 20,900,000				$ 20,900,000			$ 20,900,000
Letters of credit in support of reclamation obligations or activities	80,000,000	$ 15,700,000			80,000,000	$ 15,700,000		80,000,000	$ 126,600,000
Charge to Patriot credit support									34,700,000
Correction of error in Patriot credit support liability									16,600,000
Potential exposure from Patriot bankruptcy	$ 150,000,000				$ 150,000,000			$ 150,000,000
Liability recorded related to Patriot Bankruptcy								123.3
Portion of Patriot Liability Paid								0.7
estimated fund onbligation													$ 40,000,000
Patriot charge to loss from discontinued operations								$ (57,600,000)	(182,200,000)	$ (23,800,000)
Amount paid to the Combined Benefit Fund								2.6
Combined Benefit Fund Lower Estimate								$ 2,000,000
Combined Benefit Fund Future Estimate								$ 3,000,000
Liability related to Combined Benefit Fund								22.7
Number of VEBA payments from Patriot settlement	4				4			4
Initial Payment based on the negotiated settlement												$ 90,000,000
Payment to Patriot based on the construct of the negotiated settlement												$ 70,000,000
Payment to the VEBA based on the construct of the negotiated settlement								$ 75,000,000	75,000,000	20,000,000
Gain on VEBA settlement						68,100,000		$ 68,100,000	0	0
Number of annual payments to VEBA	5				5			5
Payment to VEBA for start up and administrative costs								$ 100,000
Withdrawal liability related to pension plan		767,000,000				$ 767,000,000
Loss from discontinued operations, net of income taxes	$ 13,100,000	$ 38,100,000	$ 155,100,000	$ 7,600,000	$ (53,600,000)			(57,600,000)	(175,000,000)	$ (28,200,000)
Scenario, Forecast [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Funding of the newly established VEBA											$ 310,000,000
Payment to the VEBA based on the construct of the negotiated settlement							$ 70,000,000
Combined benefit fund [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Patriot charge to loss from discontinued operations								1,200,000	24,600,000
Retiree Health Care Obligations [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								$ 16,100,000
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	$ 54,300,000
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									7,500,000
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									5,000,000
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									10,000,000
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									15,000,000
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									$ 15,000,000