Annual report pursuant to Section 13 and 15(d)

Financial Instruments - Effect on Other Comprehensive Income (Details)

v3.10.0.1
Financial Instruments - Effect on Other Comprehensive Income (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of gain (loss) recognized in OCI on derivatives $ (344) $ (6,060) $ 3,448
Amount of gain (loss) reclassified from accumulated OCI into income 97 (262) (1,433)
Interest rate caps      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of gain (loss) recognized in OCI on derivatives (981) (4,760) 4,250
Interest rate caps | Interest expense      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of gain (loss) reclassified from accumulated OCI into income (256) (40) 0
Natural gas swaps      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of gain (loss) recognized in OCI on derivatives 637 (1,300) (802)
Natural gas swaps | Cost of goods sold      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of gain (loss) reclassified from accumulated OCI into income $ 353 $ (222) $ (1,433)