Significant Accounting Policies - Cash, Cash Equivalent, and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 37,575	$ 30,284	$ 46,156
Restricted cash	126,544	144,930	136,667
Total cash, cash equivalents and restricted cash	$ 164,119	$ 175,214	$ 182,823	$ 250,241