Discontinued Operations (Details) AUD in Millions, $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2016 AUD
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss from discontinued operations, net of income taxes	$ (38.1)	$ (157.5)	$ (40.5)	$ (44.5)	$ (202.7)
Other current assets	3.9		3.9	3.9		$ 3.1
Investments and other assets	13.2		13.2	13.2		13.2
Total assets classified as discontinued operations	17.1		17.1	17.1		16.3
Accounts payable and accrued expenses	30.2		30.2	30.2		60.0
Other noncurrent liabilities	197.0		197.0	197.0		203.7
Liabilities subject to compromise	8,331.6		8,331.6	8,331.6		0.0
Total liabilities classified as discontinued operations	273.1		273.1	273.1		263.7
Wilkie Creek [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Charge to discontinued operations					$ 7.6	9.7
Settlement charges total	9.9		9.9	9.9			AUD 13.0
Accounts Payable [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Liabilities subject to compromise	45.9		45.9	45.9		$ 0.0
United Mine Workers of America [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [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