Revenue Recognition - Impacts of adoption in the current period - BS (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
CONDENSED CONSOLIDATED BALANCE SHEETS
Prepayments and other current assets	$ 35,386		$ 68,136
Total current assets	264,313		342,928
Other assets	39,444		36,774
Total assets	1,188,999		1,205,605
Advance payments and deposits	17,727		25,178
Accrued taxes	10,238		6,751
Total current liabilities	157,390		161,705
Deferred tax liability	30,755		31,732
Total liabilities	369,490		375,382
Retained earnings	550,872		552,948
Minority interest	136,424		141,496
Total Equity	819,509		830,223	$ 820,450	$ 809,169
Total liabilities and equity	1,188,999		$ 1,205,605
ASU 2014-09 | Change
CONDENSED CONSOLIDATED BALANCE SHEETS
Prepayments and other current assets	2,757
Total current assets	2,757
Other assets	1,433
Total assets	4,190
Advance payments and deposits	299
Accrued taxes	142
Total current liabilities	441
Deferred tax liability	301	$ 300
Total liabilities	742
Retained earnings	1,967	1,500
Minority interest	1,481	$ 1,100
Total Equity	3,448
Total liabilities and equity	4,190
ASU 2014-09 | Under previous guidance
CONDENSED CONSOLIDATED BALANCE SHEETS
Prepayments and other current assets	32,629
Total current assets	261,556
Other assets	38,011
Total assets	1,184,809
Advance payments and deposits	17,428
Accrued taxes	10,096
Total current liabilities	156,949
Deferred tax liability	30,454
Total liabilities	368,748
Retained earnings	548,905
Minority interest	134,943
Total Equity	816,061
Total liabilities and equity	$ 1,184,809