Fair Value Measurements and Derivative Instruments - Fuel Price Risk (Details) (Fuel Price Risk, USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Derivative Instruments
Estimated unrealized net gains associated with cash flow hedges pertaining to fuel swap agreements expected to be reclassified to earnings from accumulated other comprehensive income loss	$ 8.5	$ 9.5
Swaps 2014
Derivative Instruments
Fuel Swap Agreements (metric tons)	731,000	762,000
Percentage of projected requirements	55.00%	57.00%
Swaps 2015
Derivative Instruments
Fuel Swap Agreements (metric tons)	720,000	665,000
Percentage of projected requirements	51.00%	45.00%
Swaps 2016
Derivative Instruments
Fuel Swap Agreements (metric tons)	526,000	372,000
Percentage of projected requirements	35.00%	25.00%
Swaps 2017
Derivative Instruments
Fuel Swap Agreements (metric tons)	229,000	74,000
Percentage of projected requirements	15.00%	5.00%

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

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- Definition

The percentage of expected requirements covered by the aggregate notional amount of derivative contracts expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract as a percentage of expected need.

+ References

No definition available.

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- Definition

The estimated net amount of unrealized gains or losses on price risk cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7668309&loc=d3e80748-113994

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