Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) (Details)	12 Months Ended					48 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2017 USD ($)	Jan. 31, 2017 USD ($)	Jan. 31, 2014 USD ($)	Oct. 31, 2007 USD ($)
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Credit support to Patriot			$ 121,500,000
Credit support in the form of surety bonds related to Patriot Coal Act obligations			35,300,000
Patriot credit support assumed for underlying liabilities			8,500,000
Credit support to Patriot net letters of credit			83,100,000
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
Patriot charge to loss from discontinued operations			(182,200,000)	$ (23,800,000)	$ (329,700,000)
Black Lung Liability Future Cost, Lower Estimate			10,000,000
Black Lung Liability Future Costs, Upper Estimate			15,000,000
Estimated Fund Obligation									$ 40,000,000
Combined Benefit Fund Lower Estimate			2,000,000
Combined Benefit Fund Future Estimate			$ 3,000,000
Number of VEBA payments from Patriot settlement			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	$ 20,000,000
Number of remaining VEBA payments			2
Number of annual payments to VEBA			5
Payment to VEBA for start up and administrative costs			$ 100,000
Withdrawal liability related to pension plan			767,000,000
Peabody's share of the withdrawal liability related to the pension plan			644,200,000
Arch's share of the withdrawal liability related to the pension plan			299,800,000
Surety Bond [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Credit support to Patriot			81,000,000
Standby Letters of Credit [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Credit support to Patriot			29,900,000
Credit support to Patriot in form of letters of credit			22,400,000
Corporate Guarantees [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Credit support to Patriot			18,100,000
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	$ 75,000,000
Potential escrow related to UMWA litigation							$ 18,800,000
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		$ 7,500,000
Patriot Black Lung [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Patriot charge to loss from discontinued operations			114,400,000
Previous accrued credit support			15,000,000
Combined benefit fund [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Patriot charge to loss from discontinued operations			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