Fair Value Measurements and Derivative Instruments (Details 5) (Fuel Price Risk, USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivative disclosures
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	$ 10.6	$ 47.2
Swaps 2013
Derivative disclosures
Fuel Swap Agreements (metric tons)	214,000	755,000
Percentage of projected requirements	63.00%	55.00%
Swaps 2014
Derivative disclosures
Fuel Swap Agreements (metric tons)	762,000	635,000
Percentage of projected requirements	56.00%	45.00%
Swaps 2015
Derivative disclosures
Fuel Swap Agreements (metric tons)	590,000	363,000
Percentage of projected requirements	40.00%	25.00%
Swaps 2016
Derivative disclosures
Fuel Swap Agreements (metric tons)	298,000	104,000
Percentage of projected requirements	20.00%	7.00%
Swaps 2017
Derivative disclosures
Fuel Swap Agreements (metric tons)	74,000
Percentage of projected requirements	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

+ Details

Name:	invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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.

+ Details

Name:	us-gaap_DerivativeNonmonetaryNotionalAmountPercentOfRequiredNeedCoverage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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

+ Details

Name:	us-gaap_PriceRiskCashFlowHedgeUnrealizedGainLossToBeReclassifiedDuringNext12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant