Quarterly report pursuant to Section 13 or 15(d)

Investments in Affiliated Companies - Narratives (Details)

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Investments in Affiliated Companies - Narratives (Details) - Business Combination - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Business Acquisition          
Purchase accounting fair value adjustments     $ 261,384   $ 264,700
Charges related to purchase accounting fair value adjustments $ 1,658 $ 1,658 $ 3,316 $ 5,282