Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) (Details) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended				48 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 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		$ 20.9	$ 20.9
Letters of credit in support of reclamation obligations or activities	15.7		15.7	15.7
Potential exposure from Patriot bankruptcy	150.0		150.0	150.0
Black Lung Liability, Outstanding	$ 132.9		$ 132.9	132.9
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		4	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		$ 767.0	767.0
Peabody's share of the withdrawal liability related to the pension plan as estimated by the plaintiffs	644.2		644.2	644.2
Peabody's withdrawal liability related to the pension plan claim in bankruptcy court	642.7		642.7	642.7
Loss from discontinued operations, net of income taxes	(38.1)	$ (157.5)	(40.5)	(44.5)	$ (202.7)
Accounts payable and accrued expenses	30.2		30.2	30.2				60.0
Liabilities subject to compromise	8,331.6		8,331.6	8,331.6				$ 0.0
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
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	35.0
Accounts payable and accrued expenses	10.0		10.0	10.0
Liabilities subject to compromise	$ 25.0		$ 25.0	$ 25.0