Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Details) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jul. 26, 2016 USD ($)	Apr. 18, 2016 USD ($)	Apr. 15, 2016 USD ($)	Apr. 13, 2016 USD ($)	Dec. 31, 2015 USD ($)
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		$ 1,932.0		$ 1,932.0
Letters of credit in support of reclamation obligations or activities		15.7		15.7
Letters of credit outstanding, amount		144.9		144.9
Amount in bank guarantees, letters of credit and surety bonds related to collateral for road maintenance, performance guarantees and other operations		108.9		108.9
Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Maximum capacity of the securitization program		180.0		180.0						$ 275.0
Restricted cash and cash equivalents		22.8		22.8
Total consideration received by Company related to accounts receivable sold under securitization program				2,109.6
Cash up front from sale of receivables				1,102.6
Additional cash upon collection of underlying receivables				836.4
Non-collected receivables		170.6		170.6
Reduction in accounts receivable as a result of securitization activity		0.0		0.0						168.5
Expense associated with securitization transactions		2.4	$ 0.5	5.6	$ 1.3
DTA and PBGC
Guarantee Obligations [Line Items]
Letters of credit outstanding, amount		$ 37.0		$ 37.0
Dominion Terminal Associates Partnership
Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Ownership Percentage in DTA		37.50%		37.50%
DTA lease term (years)		30		30
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		2
Letter of credit maintained by the Company in favor of the PBGC		$ 37.0		$ 37.0
Guarantee in place from TXU Europe Limited		110.0		110.0
Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		1,618.3		1,618.3
Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		96.7		96.7
Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		71.1		71.1
Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	145.9		145.9
Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		315.0		315.0
Self bonding
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		1,127.5		1,127.5
Self bonding | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		1,127.5		1,127.5
Self bonding | Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0		0.0
Self bonding | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0		0.0
Self bonding | Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	0.0		0.0
Self bonding | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0		0.0
Surety bonds
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		451.2		451.2
Surety bonds | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		320.1		320.1
Surety bonds | Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		96.7		96.7
Surety bonds | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		19.1		19.1
Surety bonds | Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	15.3		15.3
Surety bonds | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		112.6		112.6
Bank guarantees
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		196.8		196.8
Bank guarantees | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		170.7		170.7
Bank guarantees | Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0		0.0
Bank guarantees | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0		0.0
Bank guarantees | Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	26.1		26.1
Bank guarantees | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		202.4		202.4
Standby Letters of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		156.5		156.5
Standby Letters of Credit [Member] | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0		0.0
Standby Letters of Credit [Member] | Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0		0.0
Standby Letters of Credit [Member] | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		52.0		52.0
Standby Letters of Credit [Member] | Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	104.5		104.5
Standby Letters of Credit [Member] | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		0.0		0.0
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)		$ (38.4)						$ (38.4)
Scenario, Actual [Member] | Dominion Terminal Associates Partnership
Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Repayments of lines of credit		39.9
Line of Credit Assumed		$ 42.7
DIP Facility - Initially Available [Member]
Guarantee Obligations [Line Items]
Long-term debt							$ 200.0	$ 200.0	$ 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

[1] Other includes the $37.0 million in letters of credit related to TXU Europe Limited described below and an additional $108.9 million in bank guarantees, letters of credit and surety bonds related to collateral for road maintenance, performance guarantees and other operations.