Annual report pursuant to Section 13 and 15(d)

Equity Method Investments - Additional Information (Details)

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Equity Method Investments - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Equity Method Investments and Joint Ventures [Abstract]                      
Income (loss) from equity method investments $ 2,192 $ 11,671 $ 10,583 $ (2,285) $ 2,866 $ 6,876 $ 8,377 $ 4,171 $ 22,162 $ 22,289 $ 6,110