Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Details) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		$ 2,472.6		$ 2,472.6
Letters of credit outstanding, amount		120.0		120.0
Amount in bank guarantees, letters of credit and surety bonds related to collateral for road maintenance, performance guarantees and other operations		154.0		154.0
Amount in letters of credit issued as collateral to support Patriot surety bonds		81.0		81.0
Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Letter of credit arrangement for workers compensation		32.8		32.8
Collateral for letter of credit related to workers compensation		46.9		46.9
Maximum capacity of the securitization program		275.0		275.0
Amount available under the securitization program		48.2		48.2
Total consideration received by Company related to accounts receivable sold under securitization program				2,815.1
Cash up front from sale of receivables				1,860.8
Additional cash upon collection of underlying receivables				529.4
Non-collected receivables		173.5		173.5
Reduction in accounts receivable as a result of securitization activity		25.0		25.0		$ 30.0
Expense associated with securitization transactions		0.5	$ 0.4	1.3	$ 1.2
DTA and PBGC
Guarantee Obligations [Line Items]
Letters of credit outstanding, amount		$ 79.7		$ 79.7
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
Maximum reimbursement obligation to commercial bank related to Dominion Terminals Associates		$ 42.7		$ 42.7
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		2,061.7		2,061.7
Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		110.6		110.6
Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		66.6		66.6
Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	233.7		233.7
Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		121.8		121.8
Self bonding
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		1,470.0		1,470.0
Self bonding | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		1,470.0		1,470.0
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		457.8		457.8
Surety bonds | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		304.7		304.7
Surety bonds | Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		110.6		110.6
Surety bonds | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		27.8		27.8
Surety bonds | Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	14.7		14.7
Surety bonds | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[2]	40.6		40.6
Bank guarantees
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		366.1		366.1
Bank guarantees | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		287.0		287.0
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]	79.1		79.1
Bank guarantees | Financial Standby Letter of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		81.2		81.2
Standby Letters of Credit [Member]
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk		178.7		178.7
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		38.8		38.8
Standby Letters of Credit [Member] | Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	[1]	139.9		139.9
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		$ (44.7)		$ (44.7)		$ (44.7)

[1] Other includes the $79.7 million in letters of credit related to Dominion Terminal Associates and TXU Europe Limited described below and an additional $154.0 million in bank guarantees, letters of credit and surety bonds related to collateral for road maintenance, performance guarantees and other operations.

[2] A total of $81.0 million of letters of credit issued as collateral to support surety bonds related to Patriot have been excluded from above as they no longer represent off-balance sheet obligations as discussed in Note 19. "Matters Related to the Patriot Bankruptcy".