Fair Value Measurements - Summary of Valuation of the Company's Investments by Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Investments	$ 4,687	$ 5,454
Level I
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Investments	1,412	2,157
Level II
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Investments	3,275	3,297
U.S. Treasury Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Investments	3,275	3,297
U.S. Treasury Securities | Level II
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Investments	3,275	3,297
Common Stock
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Investments	1,412	2,157
Common Stock | Level I
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Investments	$ 1,412	$ 2,157