Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 16,743	$ 10,380	$ 6,449	$ 11,012
Restricted cash	984	693	924	1,078
Restricted cash for Development Projects	22,414	41,982	51,721	26,384
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 40,141	$ 53,055	$ 59,094	$ 38,474