Asset Retirement Obligations Asset Retirement Obligations (Narrative) (Details) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
asset retirement obligations [Line Items]
Liabilities settled	$ (68,698,000)	$ (29,000,000)
Surety bonds outstanding	262,900,000
Letters of Credit Outstanding, Amount	18,000,000
Asset Retirement Obligations [Member]
asset retirement obligations [Line Items]
Self-bonding obligations	$ 388,400,000

Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 143

-Paragraph 22

-Subparagraph c(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_AssetRetirementObligationLiabilitiesSettled
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration