Revenue Recognition - Impacts of adoption in the current period - BS (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
CONDENSED CONSOLIDATED BALANCE SHEETS
Prepayments and other current assets	$ 33,450		$ 68,136
Total current assets	303,355		342,928
Other assets	37,060		36,774
Total assets	1,196,818		1,205,605
Advance payments and deposits	26,750		25,178
Accrued taxes	12,067		6,751
Total current liabilities	168,891		161,705
Deferred tax liability	30,945		31,732
Total liabilities	382,803		375,382
Retained earnings	546,368		552,948
Minority interest	133,408		141,496
Total Equity	814,015		830,223	$ 813,953	$ 809,169
Total liabilities and equity	1,196,818		$ 1,205,605
Change
CONDENSED CONSOLIDATED BALANCE SHEETS
Retained earnings		$ 1,500
Minority interest		1,100
ASU 2014-09 | Change
CONDENSED CONSOLIDATED BALANCE SHEETS
Prepayments and other current assets	2,426
Total current assets	2,426
Other assets	1,298
Total assets	3,724
Advance payments and deposits	277
Accrued taxes	93
Total current liabilities	370
Deferred tax liability	301	$ 300
Total liabilities	671
Retained earnings	1,725
Minority interest	1,328
Total Equity	3,053
Total liabilities and equity	3,724
ASU 2014-09 | Under previous guidance
CONDENSED CONSOLIDATED BALANCE SHEETS
Prepayments and other current assets	31,024
Total current assets	300,929
Other assets	35,762
Total assets	1,193,094
Advance payments and deposits	26,473
Accrued taxes	11,974
Total current liabilities	168,521
Deferred tax liability	30,644
Total liabilities	382,132
Retained earnings	544,643
Minority interest	132,080
Total Equity	810,962
Total liabilities and equity	$ 1,193,094