Annual report pursuant to Section 13 and 15(d)

Goodwill and Other Intangible Assets - Schedule of changes in the carrying amount of goodwill (Details)

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Goodwill and Other Intangible Assets - Schedule of changes in the carrying amount of goodwill (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Goodwill                      
Beginning balance       $ 306,691       $ 303,875 $ 306,691 $ 303,875  
Goodwill resulting from business combination and purchase price adjustments                   2,816  
Goodwill impairment $ 0 $ 0 $ (131,997) $ 0 $ 0 $ 0 $ 0 $ 0 (131,997) 0 $ 0
Goodwill related to disposed assets (103,474)               (103,474)    
Effect of currency translation adjustment                 23    
Ending balance $ 71,243       $ 306,691       $ 71,243 $ 306,691 $ 303,875