Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012	Dec. 31, 2012	Oct. 31, 2007 T
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	$ 2,267.5
Amount in letters of credit and surety bonds related to collateral for surety companies, road maintenance, performance guarantees and other operations	178.2
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Maximum capacity of the securitization program	275.0
Amount available under the securitization program	195.3
Total consideration received by Company related to accounts receivable sold under securitization program	1,042.9
Cash up front from sale of receivables	624.5
Additional cash upon collection of underlying receivables	167.0
Non collected receivables	251.4
Accounts receivable from securitization	25.0			25.0
Expense associated with securitization transactions	0.3		0.5
Number of company operated mines included in Patriot Spin-off					8
Number of majority owned JV mines included in the Patriot spin-off					2
Number of coal preparation facilities servicing contractor-operated mines included in the Patriot spin-off					8
Proven and probable coal reserves related to Patriot					1,200,000,000
Potential exposure from Patriot bankruptcy	150
Patriot collateral posted with the Department of Labor				15
DTA and PBGC
Guarantee Obligations [Line Items]
Letters of credit outstanding, amount	79.7
Dominion Terminal Associates Partnership
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Ownership Percentage in DTA	37.50%
DTA lease term	30
Number of letters of credit supporting reimbursement obligation	4
Maximum reimbursement obligation to commercial bank related to Dominion Terminals	42.7
Pension Plans Agreement With PBG and TXU Europe
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (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]
Guarantor obligations, current carrying value	1,841.0
Lease Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	105.1
Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	63.5
Other
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	257.9	[1]
Self bonding
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	1,242.5
Self bonding | Reclamation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	1,242.5
Self bonding | Lease Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0
Self bonding | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0
Self bonding | Other
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0	[1]
Surety bonds
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	461.3
Surety bonds | Reclamation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	316.4
Surety bonds | Lease Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	105.1
Surety bonds | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	30.9
Surety bonds | Other
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	8.9	[1]
Bank guarantees
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	451.4
Bank guarantees | Reclamation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	282.1
Bank guarantees | Lease Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0
Bank guarantees | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0
Bank guarantees | Other
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	169.3	[1]
Letters of credit
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	112.3
Letters of credit | Reclamation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0
Letters of credit | Lease Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	0
Letters of credit | Workers' Compensation Obligations
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	32.6
Letters of credit | Other
Guarantee Obligations [Line Items]
Guarantor obligations, current carrying value	$ 79.7	[1]

[1] Other includes the $79.7 million in letters of credit described below and an additional $178.2 million in bank guarantees and surety bonds related to collateral for surety companies, road maintenance, performance guarantees and other operations.