Asset Retirement Obligations (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
asset retirement obligations [Line Items]
Letters of Credit Outstanding, Amount	$ 18.1
Asset retirement obligations [Member]
asset retirement obligations [Line Items]
Self-bonding obligations	417.6
Asset retirement obligations [Member]
asset retirement obligations [Line Items]
Surety bonds outstanding	$ 247.3

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

No definition available.

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant