Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) (Details)	3 Months Ended		9 Months Ended		12 Months Ended				48 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	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]
Total credit support to Patriot	$ 121,500,000		$ 121,500,000
Credit support in the form of surety bonds related to Patriot's Coal Act obligations	35,300,000		35,300,000
Credit support to Patriot net letters of credit	99,100,000		99,100,000
Charge to Patriot credit support			34,700,000
Correction of error in Patriot credit support liability	16,600,000		16,600,000
Potential exposure from Patriot bankruptcy	150,000,000		150,000,000
Charge related to Patriot credit support	$ (157,500,000)	$ 7,600,000	(202,700,000)	$ 8,600,000
Black Lung Liability Future Cost, Lower Estimate			10,000,000
Black Lung Liability Future Costs, Upper Estimate			15,000,000
Estimated fund obligations											$ 40,000,000
Combined Benefit Fund Lower Estimate			2,000,000
Combined Benefit Fund Upper Estimate			$ 3,000,000
Number of VEBA payments from Patriot settlement	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			$ 16,100,000					$ 20,000,000
Number of remaining VEBA payments	2		2
Number of annual payments to VEBA	5		5
VEBA start up costs			$ 100,000
Accrued liabilities related to Patriot	$ 33,600,000		$ 33,600,000
Number of days to file declaratory judgment	30		30
Withdrawal liability related to pension plan	$ 767,000,000		$ 767,000,000
Peabody's share of the withdrawal liability related to the pension plan as estimated by the plaintiffs	644,200,000		644,200,000
Arch's share of the withdrawal liability related to the pension plan as estimated by the plaintiffs	299,800,000		299,800,000
Pension Plan Annual Cost Estimate			22,500,000
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	$ 75,000,000
Surety Bond [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Total credit support to Patriot	81,000,000		81,000,000
Standby Letters of Credit [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Total credit support to Patriot	22,400,000		22,400,000
Corporate Guarantees [Member]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Total credit support to Patriot	$ 18,100,000		18,100,000
Patriot Black Lung [Domain]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Charge related to Patriot credit support			126,000,000
Previously accrued credit support			15,000,000
UMWA combined benefit fund [Domain]
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items]
Charge related to Patriot credit support			$ 29,100,000