Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Details) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Jul. 26, 2016 USD ($)	Apr. 15, 2016 USD ($)
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		$ 1,764.9
Cash Collateral		89.2
Letters of credit in support of reclamation obligations or activities		15.7
Letters of credit outstanding, amount		167.2
Amount in bank guarantees, letters of credit and surety bonds related to collateral for road maintenance, performance guarantees and other operations		65.7
Amount in letters of credit issued as collateral to support Patriot surety bonds		72.6
Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Successor Notes		1,000.0
Restricted cash and cash equivalents		66.9
Total consideration received by Company related to accounts receivable sold under securitization program		724.7
Cash up front from sale of receivables		403.6
Additional cash upon collection of underlying receivables		55.7
Non-collected receivables		265.4
Expense associated with securitization transactions		2.5	$ 0.8
Restricted Cash and Investments, Current		80.7		$ 54.3
Restricted Cash and Investments, Noncurrent		88.0		71.4
Securitization Purchase Limit		250.0
Collateral Posted, Mining and Reclamation Obligations		506.0		457.9
DTA and PBGC
Guarantee Obligations [Line Items]
Letters of credit outstanding, amount		$ 37.0
Pension Plans Agreement With PBGC and TXU Europe
Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Defined benefit pension plans requiring special contributions		2
Letter of credit maintained by the Company in favor of the PBGC		$ 37.0
Guarantee in place from TXU Europe Limited		110.0
Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		1,508.6
Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		93.8
Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		59.8
Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	102.7
Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		364.6
Self bonding
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		894.7
Self bonding | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		894.7
Self bonding | Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0
Self bonding | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0
Self bonding | Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	0.0
Self bonding | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0
Surety bonds
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		475.3
Surety bonds | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		348.4
Surety bonds | Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		93.8
Surety bonds | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		19.1
Surety bonds | Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	14.0
Surety bonds | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		73.4
Bank guarantees
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		83.2
Bank guarantees | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		57.8
Bank guarantees | Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0
Bank guarantees | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0
Bank guarantees | Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	25.4
Bank guarantees | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		83.5
Standby Letters of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		311.7
Standby Letters of Credit [Member] | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		207.7
Standby Letters of Credit [Member] | Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0
Standby Letters of Credit [Member] | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		40.7
Standby Letters of Credit [Member] | Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	63.3
Standby Letters of Credit [Member] | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		207.7
Performance guarantee for third-party coal producer
Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Carrying amount of guarantee liability for reclamation and bonding commitments				$ (38.4)
DIP Facility - Initially Available [Member]
Guarantee Obligations [Line Items]
Long-term debt						$ 200.0
Wyoming [Member] | Self bonding | Reclamation Obligations
Guarantee Obligations [Line Items]
Letters of credit in support of reclamation obligations or activities					$ 0.8
Indiana [Member] | Self bonding | Reclamation Obligations
Guarantee Obligations [Line Items]
Letters of credit in support of reclamation obligations or activities					7.5
Illinois [Member] | Self bonding | Reclamation Obligations
Guarantee Obligations [Line Items]
Letters of credit in support of reclamation obligations or activities					$ 3.2
U.S Mining [Member]
Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Reclamation bonding requirements		1,243.1
Costs Incurred, Asset Retirement Obligation Incurred		476.7
Australian Mining [Member]
Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Costs Incurred, Asset Retirement Obligation Incurred		$ 271.3

[1] Other includes the $37.0 million in letters of credit related to TXU Europe Limited described below and an additional $65.7 million in bank guarantees, letters of credit and surety bonds related to collateral for road maintenance, performance guarantees and other operations.