Revenue Recognition - Impacts of adoption in the current period - (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 02, 2018	Jan. 01, 2018	Dec. 31, 2017
Contracts
Contract asset, current	$ 1,309	$ 1,176
Contract asset, noncurrent	498	453
Contract liability	9,827	$ 9,912
Deferred tax liability	30,945			$ 31,732
Retained earnings	546,368			552,948
Minority interest	133,408			$ 141,496
Prepayments and other current assets
Contracts
Contract acquisition costs, current	1,100
Other assets
Contracts
Contract acquisition costs, noncurrent	800
Change
Contracts
Retained earnings			$ 1,500
Minority interest			1,100
Change | ASU 2014-09
Contracts
Contract asset			1,600
Contract acquisition costs			1,500
Deferred tax liability	301		300
Retained earnings	1,725
Minority interest	$ 1,328
Change | ASU 2014-09 | Prepayments and other current assets
Contracts
Contract asset, current			1,200
Contract acquisition costs, current			900
Change | ASU 2014-09 | Other assets
Contracts
Contract asset, noncurrent			400
Retained earnings			600
Change | ASU 2014-09 | Advance Payments And Deposits
Contracts
Contract liability			$ 200