Revenue Recognition - Impacts of adoption in the current period - (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Contracts
Contract asset, current	$ 1,434	$ 1,309
Contract asset, noncurrent	568	498
Contract liability	9,552	$ 9,827
Deferred tax liability	30,755			$ 31,732
Retained earnings	550,872			552,948
Minority interest	136,424			$ 141,496
Prepayments and other current assets
Contracts
Contract acquisition costs, current	1,300
Other assets
Contracts
Contract acquisition costs, noncurrent	900
Change | ASU 2014-09
Contracts
Contract asset			$ 1,600
Contract acquisition costs			1,500
Deferred tax liability	301		300
Retained earnings	1,967		1,500
Minority interest	$ 1,481		1,100
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