Summary of Significant Accounting Policies - Reconciliation of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 28,338				$ 10,380	$ 11,012
Restricted cash	373				693	1,078
Restricted cash and cash equivalents for Development Projects	21,039				41,982	26,384
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$ 49,750	$ 83,070	$ 40,141	$ 64,643	$ 53,055	$ 38,474	$ 23,225