Financial Instruments, Guarantees with Off-Balance-Sheet Risk and Other Guarantees (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013	Sep. 30, 2014		Sep. 30, 2013	Dec. 31, 2013
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	$ 2,457.3			$ 2,457.3
Amount in bank guarantees, letters of credit and surety bonds related to collateral for road maintenance, performance guarantees and other operations	137.5			137.5
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	247.4			247.4
Total consideration received by Company related to accounts receivable sold under securitization program				3,196.8
Cash up front from sale of receivables				2,064.8
Additional cash upon collection of underlying receivables				918.0
Non-collected receivables	214.0			214.0
Reduction to accounts receivable from securitization	25.0			25.0			100.0
Expense associated with securitization transactions	0.4		0.5	1.2		1.2
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
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]
Financial instruments with off-balance sheet risk	2,003.5			2,003.5
Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	104.4			104.4
Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	132.2			132.2
Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	217.2	[1]		217.2	[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
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]
Financial instruments with off-balance sheet risk	1,374.3			1,374.3
Self bonding | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	1,374.3			1,374.3
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	0	[1]		0	[1]
Surety bonds
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	539.3			539.3
Surety bonds | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	326.2			326.2
Surety bonds | Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	104.4			104.4
Surety bonds | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	98.1			98.1
Surety bonds | Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	10.6	[1]		10.6	[1]
Bank guarantees
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	406.4			406.4
Bank guarantees | Reclamation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	285.4			285.4
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	121.0	[1]		121.0	[1]
Letters of credit
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	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]
Financial instruments with off-balance sheet risk	17.6			17.6
Letters of credit | Lease Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	0			0
Letters of credit | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	34.1			34.1
Letters of credit | Other
Guarantee Obligations [Line Items]
Financial instruments with off-balance sheet risk	85.6	[1]		85.6	[1]
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	$ 45.6			$ 45.6			$ 45.6

[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 $137.5 million in bank guarantees, letters of credit and surety bonds related to collateral for road maintenance, performance guarantees and other operations.