Commitments, Guarantees and Contingencies - Coal, Rail and Shipping Contracts (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012
Coal, Rail and Shipping Contracts [Line Items]
Coal Contracts, Expiration Date	2013
Transportation Contracts, Expiration Date	2015
Coal and Transportation [Member]
Coal, Rail and Shipping Contracts [Line Items]
Minimum Annual Payment Obligation, Current Year	37.4
Minimum Annual Payment Obligation, Year Two	23.8

The aggregate amount of payments due in the second year after the balance sheet date to reduce a recorded unconditional purchase obligation.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 47

-Paragraph 10

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 440

-SubTopic 10

-Section 50

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6394976&loc=d3e25383-109308

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