Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013	Jun. 30, 2014		Jun. 30, 2013	Dec. 31, 2013
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	$ 2,463.2			$ 2,463.2
Amount in bank guarantees, letters of credit and surety bonds related to collateral for road maintenance, performance guarantees and other operations	157.7			157.7
Financial Instruments Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Letter of credit arrangement for workers compensation	22.5			22.5
Collateral for letter of credit related to workers compensation	32.1			32.1
Maximum capacity of the securitization program	275.0			275.0
Amount available under the securitization program	94.6			94.6
Total consideration received by Company related to accounts receivable sold under securitization program				2,108.5
Cash up front from sale of receivables				1,403.1
Additional cash upon collection of underlying receivables				575.1
Non-collected receivables	130.3			130.3
Reduction to accounts receivable from securitization	110.0			110.0			100.0
Expense associated with securitization transactions	0.4		0.4	0.8		0.7
Credit support to Patriot	135.7			135.7
Credit support to Patriot ending in 2018	100			100
Credit support in the form of surety bonds related to Patriots Coal Act obligations	50.7			50.7
Potential exposure from Patriot bankruptcy	150			150
Liability for reclamation and bonding commitments provided on behalf of third-party coal producer	45.6			45.6			45.6
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	30			30
Number of letters of credit supporting reimbursement obligation				4
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]
Guarantor obligations, current carrying value	1,988.5			1,988.5
Lease Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	105.1			105.1
Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	132.2			132.2
Other
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	237.4	[1]		237.4	[1]
Surety bond
Financial Instruments Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Credit support to Patriot	81.4			81.4
Corporate Guarantees [Member]
Financial Instruments Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Credit support to Patriot	31.9			31.9
Self bonding
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	1,366.3			1,366.3
Self bonding | Reclamation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	1,366.3			1,366.3
Self bonding | Lease Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0			0
Self bonding | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0			0
Self bonding | Other
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0	[1]		0	[1]
Surety bonds
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	542.3			542.3
Surety bonds | Reclamation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	328.1			328.1
Surety bonds | Lease Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	105.1			105.1
Surety bonds | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	98.1			98.1
Surety bonds | Other
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	11.0	[1]		11.0	[1]
Bank guarantees
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	417.3			417.3
Bank guarantees | Reclamation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	276.5			276.5
Bank guarantees | Lease Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0			0
Bank guarantees | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0			0
Bank guarantees | Other
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	140.8	[1]		140.8	[1]
Letters of credit
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	137.3			137.3
Financial Instruments Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Textuals) [Abstract]
Credit support to Patriot	22.4			22.4
Letters of credit | Reclamation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	17.6			17.6
Letters of credit | Lease Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0			0
Letters of credit | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	34.1			34.1
Letters of credit | Other
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	$ 85.6	[1]		$ 85.6	[1]

[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 $157.7 million in bank guarantees, letters of credit and surety bonds related to collateral for road maintenance, performance guarantees and other operations.