Commitments, Guarantees and Contingencies - Coal, Rail and Shipping Contracts (Details) (Coal Supply and Transportation [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Coal Supply and Transportation [Member]
Coal, Rail and Shipping Contracts [Line Items]
Coal Supply and Transportation Agreements, Minimum Annual Payment Obligation for Remainder of 2015	$ 42.3
Coal Supply and Transportation Agreements, Minimum Annual Payment Obligation in 2016	27.0
Coal Supply and Transportation Agreements, Minimum Annual Payment Obligation in 2017	27.6
Coal Supply and Transportation Agreements, Minimum Annual Payment Obligation in 2018	$ 28.4

Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 440

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6394976&loc=d3e25336-109308

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- Definition

Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- Definition

Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- Definition

Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the remainder of the fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

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