Annual report pursuant to Section 13 and 15(d)

Derivative Instruments and Hedging Activities - Effects on statements of income and consolidated statements of comprehensive income (Details)

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Derivative Instruments and Hedging Activities - Effects on statements of income and consolidated statements of comprehensive income (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) on derivatives recognized in net income (loss) $ (1,068) $ (121) $ (271)
Nonoperating Income (Expense) | Designated as Hedging Instrument | Interest Rate Swap      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) on derivatives recognized in net income (loss) 71 (196) (271)
Nonoperating Income (Expense) | Not Designated as Hedging Instrument | Interest Rate Swap      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) on derivatives recognized in net income (loss) 56 (308) 0
Nonoperating Income (Expense) | Not Designated as Hedging Instrument | Commodity swap contracts      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) on derivatives recognized in net income (loss) (234) 36 0
Nonoperating Income (Expense) | Not Designated as Hedging Instrument | Interest make-whole provisions      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) on derivatives recognized in net income (loss) $ (890) $ 337 $ 0