Basis of Preparation and Critical Accounting Policies - Cash, cash equivalents and restricted cash (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Basis of Preparation and Critical Accounting Policies
Cash and cash equivalents	$ 259,530	$ 292,204	$ 239,710
Restricted cash	21,528	16,078	22,239
Total cash, cash equivalents and restricted cash in the consolidated statements of cash flows	$ 281,058	$ 308,282	$ 261,949	$ 290,802