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Debt (Details 4) (USD $)
Dec. 31, 2014
item
Debt  
Number of letter of credit facilities 11nbr_NumberOfLineOfCreditFacility
Letter of Credit  
Debt  
Credit available 650,204us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_LetterOfCreditMember
Letters of credit outstanding, inclusive of financial and performance guarantees 326,650us-gaap_LineOfCreditFacilityAmountOutstanding
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_LetterOfCreditMember
Remaining availability 323,554us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_LetterOfCreditMember