Supplemental Guarantor/Non-Guarantor Financial Information (Details 1) - USD ($) $ in Millions	Sep. 30, 2016	Apr. 12, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Cash and Cash Equivalents, at Carrying Value	$ 1,167.8	$ 652.1	$ 261.3	$ 334.3	$ 298.0
Restricted Cash and Investments, Current	24.2		0.0
Accounts receivable, net	351.9		228.8
Accounts Receivable, Related Parties, Current	0.0		0.0
Inventory, Net	259.6		307.8
Assets From Coal Trading Activities, Net	19.9		23.5
Deferred Tax Assets, Net of Valuation Allowance, Current	53.5		53.5
Other current assets	368.9		447.6
Assets, Current	2,245.8		1,322.5
Property, Plant and Equipment, Net	9,215.5		9,258.5
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	2.3		2.2
Investments and other assets	771.6		363.7
Notes Receivable, Related Parties	0.0		0.0
Total assets	12,235.2		10,946.9
Current portion of long-term debt	493.1		5,874.9
Accounts Payable, Related Parties, Current	0.0		0.0
Deferred Tax Liabilities, Net, Current	4.1		3.8
Liabilities from coal trading activities, net	19.5		15.6
Accounts Payable and Accrued Liabilities, Current	835.9		1,446.3
Accounts Payable and Other Accrued Liabilities, Current	831.8		1,442.5
Liabilities, Current	1,348.5		7,336.8
Long-term debt, less current portion	0.0		366.3
Deferred income taxes	107.6		69.1
Notes Payable, Related Parties, Noncurrent	0.0		0.0
Other Noncurrent Liability	1,923.6		2,256.2
Total liabilities not subject to compromise	3,379.7		10,028.4
Liabilities subject to compromise	8,331.6		0.0
Total liabilities	11,711.3		10,028.4
Stockholders' Equity Attributable to Parent	520.7		916.9
Stockholders' Equity Attributable to Noncontrolling Interest	3.2		1.6
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	523.9		918.5
Liabilities and Equity	12,235.2		10,946.9
Parent Company [Member]
Cash and Cash Equivalents, at Carrying Value	687.5		7.2	144.2	188.7
Restricted Cash and Investments, Current	1.4
Accounts receivable, net	0.0		0.0
Accounts Receivable, Related Parties, Current	1,035.2		582.1
Inventory, Net	0.0		0.0
Assets From Coal Trading Activities, Net	0.0		0.0
Deferred Tax Assets, Net of Valuation Allowance, Current	0.0		0.0
Other current assets	22.1		23.1
Assets, Current	1,746.2		612.4
Property, Plant and Equipment, Net	0.0		0.0
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	0.0		0.0
Investments and other assets	8,761.5		8,476.2
Notes Receivable, Related Parties	0.0		0.0
Total assets	10,507.7		9,088.6
Current portion of long-term debt	470.5		5,844.0
Accounts Payable, Related Parties, Current	0.0		0.0
Deferred Tax Liabilities, Net, Current	11.8		11.8
Liabilities from coal trading activities, net	0.0		0.0
Accounts Payable and Accrued Liabilities, Current	47.5		494.8
Liabilities, Current	529.8		6,350.6
Long-term debt, less current portion			366.3
Deferred income taxes	165.8		98.6
Notes Payable, Related Parties, Noncurrent	1,032.6		1,032.6
Other Noncurrent Liability	154.8		323.6
Total liabilities not subject to compromise	1,883.0
Liabilities subject to compromise	8,104.0
Total liabilities	9,987.0		8,171.7
Stockholders' Equity Attributable to Parent	520.7		916.9
Stockholders' Equity Attributable to Noncontrolling Interest	0.0		0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	520.7		916.9
Liabilities and Equity	10,507.7		9,088.6
Guarantor Subsidiaries [Member]
Cash and Cash Equivalents, at Carrying Value	3.1		4.7	4.2	3.6
Restricted Cash and Investments, Current	0.0
Accounts receivable, net	22.4		12.1
Accounts Receivable, Related Parties, Current	0.0		0.0
Inventory, Net	86.7		109.4
Assets From Coal Trading Activities, Net	0.1		3.2
Deferred Tax Assets, Net of Valuation Allowance, Current	65.3		65.3
Other current assets	107.9		128.1
Assets, Current	285.5		322.8
Property, Plant and Equipment, Net	4,419.3		4,304.8
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	63.2		33.1
Investments and other assets	3.7		3.6
Notes Receivable, Related Parties	877.0		632.7
Total assets	5,648.7		5,297.0
Current portion of long-term debt	20.5		23.8
Accounts Payable, Related Parties, Current	1,668.6		1,530.2
Deferred Tax Liabilities, Net, Current	0.0		0.0
Liabilities from coal trading activities, net	0.1		4.8
Accounts Payable and Accrued Liabilities, Current	380.6		479.8
Liabilities, Current	2,069.8		2,038.6
Long-term debt, less current portion			0.0
Deferred income taxes	0.0		0.0
Notes Payable, Related Parties, Noncurrent	0.0		0.0
Other Noncurrent Liability	1,277.7		1,454.9
Total liabilities not subject to compromise	3,347.5
Liabilities subject to compromise	209.0
Total liabilities	3,556.5		3,493.5
Stockholders' Equity Attributable to Parent	2,092.2		1,803.5
Stockholders' Equity Attributable to Noncontrolling Interest	0.0		0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,092.2		1,803.5
Liabilities and Equity	5,648.7		5,297.0
Non-Guarantor Subsidiaries [Member]
Cash and Cash Equivalents, at Carrying Value	477.2		249.4	$ 185.9	$ 105.7
Restricted Cash and Investments, Current	22.8
Accounts receivable, net	329.5		216.7
Accounts Receivable, Related Parties, Current	633.4		948.1
Inventory, Net	172.9		198.4
Assets From Coal Trading Activities, Net	19.8		20.3
Deferred Tax Assets, Net of Valuation Allowance, Current	0.0		0.0
Other current assets	238.9		296.4
Assets, Current	1,894.5		1,929.3
Property, Plant and Equipment, Net	4,796.2		4,953.7
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	2.3		0.0
Investments and other assets	501.1		185.5
Notes Receivable, Related Parties	155.6		399.9
Total assets	7,349.7		7,468.4
Current portion of long-term debt	2.1		7.1
Accounts Payable, Related Parties, Current	0.0		0.0
Deferred Tax Liabilities, Net, Current	4.1		3.8
Liabilities from coal trading activities, net	19.4		10.8
Accounts Payable and Accrued Liabilities, Current	403.7		467.9
Liabilities, Current	429.3		489.6
Long-term debt, less current portion			0.0
Deferred income taxes	5.0		1.4
Notes Payable, Related Parties, Noncurrent	0.0		0.0
Other Noncurrent Liability	491.1		477.7
Total liabilities not subject to compromise	925.4
Liabilities subject to compromise	18.6
Total liabilities	944.0		968.7
Stockholders' Equity Attributable to Parent	6,402.5		6,498.1
Stockholders' Equity Attributable to Noncontrolling Interest	3.2		1.6
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	6,405.7		6,499.7
Liabilities and Equity	7,349.7		7,468.4
Consolidation, Eliminations [Member]
Cash and Cash Equivalents, at Carrying Value	0.0		0.0
Restricted Cash and Investments, Current	0.0
Accounts receivable, net	0.0		0.0
Accounts Receivable, Related Parties, Current	(1,668.6)		(1,530.2)
Inventory, Net	0.0		0.0
Assets From Coal Trading Activities, Net	0.0		0.0
Deferred Tax Assets, Net of Valuation Allowance, Current	(11.8)		(11.8)
Other current assets	0.0		0.0
Assets, Current	(1,680.4)		(1,542.0)
Property, Plant and Equipment, Net	0.0		0.0
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	(63.2)		(30.9)
Investments and other assets	(8,494.7)		(8,301.6)
Notes Receivable, Related Parties	(1,032.6)		(1,032.6)
Total assets	(11,270.9)		(10,907.1)
Current portion of long-term debt	0.0		0.0
Accounts Payable, Related Parties, Current	(1,668.6)		(1,530.2)
Deferred Tax Liabilities, Net, Current	(11.8)		(11.8)
Liabilities from coal trading activities, net	0.0		0.0
Accounts Payable and Accrued Liabilities, Current	0.0		0.0
Liabilities, Current	(1,680.4)		(1,542.0)
Long-term debt, less current portion			0.0
Deferred income taxes	(63.2)		(30.9)
Notes Payable, Related Parties, Noncurrent	(1,032.6)		(1,032.6)
Other Noncurrent Liability	0.0		0.0
Total liabilities not subject to compromise	(2,776.2)
Liabilities subject to compromise	0.0
Total liabilities	(2,776.2)		(2,605.5)
Stockholders' Equity Attributable to Parent	(8,494.7)		(8,301.6)
Stockholders' Equity Attributable to Noncontrolling Interest	0.0		0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(8,494.7)		(8,301.6)
Liabilities and Equity	$ (11,270.9)		$ (10,907.1)