Derivative Activities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended	1 Months Ended
Dec. 31, 2012	Dec. 31, 2012 Commodity Contract [Member]	Jul. 31, 2012 Interest Rate Contract [Member]
Derivative [Line Items]
Realized loss on early termination of derivative contract		$ 5.3	$ 15.0
Deferred premiums associated with natural gas derivative instruments	4.9
Maximum loss on credit risk derivative contract	$ 123.4

The aggregate net gain (loss) from increase (decrease) in fair value of credit risk derivatives included in earnings.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 45

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959

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