Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) (Details) $ in Millions	3 Months Ended	6 Months Ended		12 Months Ended				48 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 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 ($)
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
Charge to Patriot credit support	$ 34.7
Correction of error in Patriot credit support liability	$ 16.6		$ 16.6
Potential exposure from Patriot bankruptcy		150.0
Black Lung Liability, Outstanding		131.7
Black Lung Liability Future Cost, Lower Estimate		10.0
Black Lung Liability Future Costs, Upper Estimate		$ 15.0
Number of VEBA payments from Patriot settlement		4
Initial payment based on the negotiated settlement									$ 90.0
Payment to Patriot based on the construct of the negotiated settlement									$ 70.0
Payment to the VEBA based on the construct of the negotiated settlement		$ 75.0				$ 75.0	$ 20.0
Gain on VEBA settlement		68.1	$ 0.0
Withdrawal liability related to pension plan		767.0
Peabody's share of the withdrawal liability related to the pension plan as estimated by the plaintiffs		644.2
Scenario, Forecast [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Funding of the newly established VEBA								$ 310.0
Payment to the VEBA based on the construct of the negotiated settlement				$ 70.0	$ 75.0
Scenario, Forecast (Monthly) [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