Supplemental Guarantor/Non-Guarantor Financial Information (Details 5) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents, at Carrying Value	$ 1,068.1	$ 872.3	$ 745.6	$ 261.3
Restricted Cash and Investments, Current	80.7	54.3
Restricted Cash and Cash Equivalents, Current	1,000.0	0.0
Accounts receivable, net	312.1	473.0
Accounts Receivable, Related Parties, Current	0.0	0.0
Inventory, Net	250.8	203.7
Assets From Coal Trading Activities, Net	0.6	0.7
Other current assets	493.9	486.6
Assets, Current	3,206.2	2,090.6
Property, Plant and Equipment, Net	8,653.9	8,776.7
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	0.0	0.0
Investments and other assets	976.4	910.4
Notes Receivable, Related Parties	0.0	0.0
Total assets	12,836.5	11,777.7
Current portion of long-term debt	18.2	20.2
Accounts Payable, Related Parties, Current	0.0	0.0
Taxes Payable, Current	8.6
Liabilities from coal trading activities, net	0.7	1.2
Accounts Payable and Other Accrued Liabilities, Current	958.7
Liabilities, Current	986.2	1,011.8
Long-term debt, less current portion	950.5	0.0
Deferred income taxes	179.2	173.9
Notes Payable, Related Parties, Noncurrent	0.0	0.0
Other Noncurrent Liability	1,972.0	1,970.3
Total liabilities not subject to compromise	4,087.9	3,156.0
Liabilities subject to compromise	8,416.7	8,440.2
Total liabilities	12,504.6	11,596.2
Stockholders' Equity Attributable to Parent	319.6	173.9
Stockholders' Equity Attributable to Noncontrolling Interest	12.3	7.6
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	331.9	181.5
Liabilities and Equity	12,836.5	11,777.7
Parent Company [Member]
Cash and Cash Equivalents, at Carrying Value	221.4	266.6	$ 636.9	$ 7.2
Restricted Cash and Investments, Current	13.8	13.8
Restricted Cash and Cash Equivalents, Current	1,000.0
Accounts receivable, net	0.0	0.0
Accounts Receivable, Related Parties, Current	878.2	899.9
Inventory, Net	0.0	0.0
Assets From Coal Trading Activities, Net	0.0	0.0
Other current assets	25.2	19.1
Assets, Current	2,138.6	1,199.4
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	8,825.2	8,495.7
Notes Receivable, Related Parties	0.0	0.0
Total assets	10,963.8	9,695.1
Current portion of long-term debt	0.0	0.0
Accounts Payable, Related Parties, Current	0.0	0.0
Liabilities from coal trading activities, net	0.0	0.0
Liabilities, Current	71.6	58.9
Long-term debt, less current portion	950.5
Deferred income taxes	195.4	28.0
Notes Payable, Related Parties, Noncurrent	1,032.6	1,032.5
Other Noncurrent Liability	167.3	160.4
Total liabilities not subject to compromise	2,417.4	1,279.8
Liabilities subject to compromise	8,226.8	8,241.4
Total liabilities	10,644.2	9,521.2
Stockholders' Equity Attributable to Parent	319.6	173.9
Stockholders' Equity Attributable to Noncontrolling Interest	0.0	0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	319.6	173.9
Liabilities and Equity	10,963.8	9,695.1
Debtor Subsidiaries [Member]
Cash and Cash Equivalents, at Carrying Value	261.6	394.5
Restricted Cash and Investments, Current	13.8
Restricted Cash and Cash Equivalents, Current	1,000.0
Accounts receivable, net	0.0
Accounts Receivable, Related Parties, Current	214.9
Inventory, Net	116.3
Assets From Coal Trading Activities, Net	0.3
Other current assets	78.4
Assets, Current	1,685.3
Property, Plant and Equipment, Net	4,787.7
Investments and other assets	4,410.3
Notes Receivable, Related Parties	1,047.5
Total assets	11,930.8
Current portion of long-term debt	18.2
Accounts Payable, Related Parties, Current	0.0
Taxes Payable, Current	0.0
Liabilities from coal trading activities, net	0.1
Accounts Payable and Other Accrued Liabilities, Current	546.7
Liabilities, Current	565.0
Long-term debt, less current portion	950.5
Deferred income taxes	10.4
Notes Payable, Related Parties, Noncurrent	0.0
Other Noncurrent Liability	1,668.6
Total liabilities not subject to compromise	3,194.5
Liabilities subject to compromise	8,416.7
Total liabilities	11,611.2
Stockholders' Equity Attributable to Parent	319.6
Stockholders' Equity Attributable to Noncontrolling Interest	0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	319.6
Liabilities and Equity	11,930.8
Non-Debtor Subsidiaries [Member]
Cash and Cash Equivalents, at Carrying Value	806.5	477.8
Restricted Cash and Investments, Current	66.9
Restricted Cash and Cash Equivalents, Current	0.0
Accounts receivable, net	330.8
Accounts Receivable, Related Parties, Current	0.0
Inventory, Net	134.5
Assets From Coal Trading Activities, Net	0.3
Other current assets	417.1
Assets, Current	1,756.1
Property, Plant and Equipment, Net	3,866.2
Investments and other assets	573.1
Notes Receivable, Related Parties	0.0
Total assets	6,195.4
Current portion of long-term debt	0.0
Accounts Payable, Related Parties, Current	214.9
Taxes Payable, Current	10.2
Liabilities from coal trading activities, net	0.6
Accounts Payable and Other Accrued Liabilities, Current	430.7
Liabilities, Current	656.4
Long-term debt, less current portion	0.0
Deferred income taxes	168.8
Notes Payable, Related Parties, Noncurrent	1,047.5
Other Noncurrent Liability	303.4
Total liabilities not subject to compromise	2,176.1
Liabilities subject to compromise	0.0
Total liabilities	2,176.1
Stockholders' Equity Attributable to Parent	4,007.0
Stockholders' Equity Attributable to Noncontrolling Interest	12.3
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	4,019.3
Liabilities and Equity	6,195.4
Debtor Non-Debtor Eliminations [Member]
Cash and Cash Equivalents, at Carrying Value	0.0
Restricted Cash and Investments, Current	0.0
Restricted Cash and Cash Equivalents, Current	0.0
Accounts receivable, net	(18.7)
Accounts Receivable, Related Parties, Current	(214.9)
Inventory, Net	0.0
Assets From Coal Trading Activities, Net	0.0
Other current assets	(1.6)
Assets, Current	(235.2)
Property, Plant and Equipment, Net	0.0
Investments and other assets	(4,007.0)
Notes Receivable, Related Parties	(1,047.5)
Total assets	(5,289.7)
Current portion of long-term debt	0.0
Accounts Payable, Related Parties, Current	(214.9)
Taxes Payable, Current	(1.6)
Liabilities from coal trading activities, net	0.0
Accounts Payable and Other Accrued Liabilities, Current	(18.7)
Liabilities, Current	(235.2)
Long-term debt, less current portion	0.0
Deferred income taxes	0.0
Notes Payable, Related Parties, Noncurrent	(1,047.5)
Other Noncurrent Liability	0.0
Total liabilities not subject to compromise	(1,282.7)
Liabilities subject to compromise	0.0
Total liabilities	(1,282.7)
Stockholders' Equity Attributable to Parent	(4,007.0)
Stockholders' Equity Attributable to Noncontrolling Interest	0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(4,007.0)
Liabilities and Equity	(5,289.7)
Noncontrolling Interest [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 12.3	$ 7.6