Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Summary of Significant Accounting Policies
Restricted cash and other assets	$ 321		$ 324
Cash and cash equivalents	107,459		112,629
Restricted Cash	311		311
Cash, cash equivalents and restricted cash	107,770	$ 80,005	112,940	$ 80,978
Tax expense (benefit)	0	$ 0
Uncertain tax positions	$ 0		$ 0
Practical Expedient, Financing Component	true