Summary of Significant Accounting Policies - Impairment of Long Lived Assets and Intangible Assets Subject to Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Impairment of Long Lived Assets, Intangible Assets and Goodwill
Asset impairment charge	$ 0	$ 0
Impairment charge related to intangible assets subject to amortization	0	0
Goodwill, Impairment Loss	0	0
impairment charge related to indefinite lived intangible assets	$ 0	$ 0