Supplemental Cash Flow Information of the Company - Reconciliation of cash and cash equivalents and restricted cash (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 49,693	$ 51,604	$ 53,069	$ 57,649
Restricted cash	6,300	119,430	0	9,149
Cash and cash equivalents and restricted cash	$ 55,993	$ 171,034	$ 53,069	$ 66,798