Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 18,390	$ 10,380	$ 6,213	$ 11,012
Restricted cash	502	693	920	1,078
Restricted cash for Development Projects		41,982	45,212	26,384
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 83,070	$ 53,055	$ 52,345	$ 38,474