Annual report pursuant to Section 13 and 15(d)

Financial Instruments - Fair Value of Derivative Assets and Liabilities (Details)

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Financial Instruments - Fair Value of Derivative Assets and Liabilities (Details) (USD $)
Dec. 31, 2012
Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative, Collateral, Right to Reclaim Cash $ 5,900,000 $ 5,500,000
Percentage of Debt Hedged by Interest Rate Derivatives 75.00%  
Non-current derivative liabilities 26,424,000 0
Notional amount of interest rate derivative 2,900,000,000  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Assets, Current 0 0
Derivative Liabilities, Current 0 0
Non-current derivative liabilities 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Assets, Current 237,000 1,951,000
Derivative Liabilities, Current 98,000 1,415,000
Non-current derivative liabilities   0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Assets, Current 0 0
Derivative Liabilities, Current 0 0
Non-current derivative liabilities 0 0
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Assets, Current 237,000 1,951,000
Derivative Liabilities, Current 98,000 1,415,000
Non-current derivative liabilities $ 26,424,000 $ 0