Fair Value Measurements - Summary of the Levels of the Fair Value Hierarchy into Which the Company's Financial Assets Fall (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Measurements
Total financial assets	$ 282,045	$ 223,267
U.S. Treasury Instruments
Fair Value Measurements
Cash and cash equivalents	110,529	39,978
Investments in securities	79,395	55,935
Government Debt Securities
Fair Value Measurements
Cash and cash equivalents		41,086
Investments in securities	1,105	39,910
Government Securities Money Market
Fair Value Measurements
Cash and cash equivalents	90,866	46,095
Common Stock
Fair Value Measurements
Investments in securities	150	263
Level 1
Fair Value Measurements
Total financial assets	108,627	8,206
Level 1 | U.S. Treasury Instruments
Fair Value Measurements
Cash and cash equivalents	100,500
Investments in securities	7,977	7,943
Level 1 | Common Stock
Fair Value Measurements
Investments in securities	150	263
Level 2
Fair Value Measurements
Total financial assets	173,418	215,061
Level 2 | U.S. Treasury Instruments
Fair Value Measurements
Cash and cash equivalents	10,029	39,978
Investments in securities	71,418	47,992
Level 2 | Government Debt Securities
Fair Value Measurements
Cash and cash equivalents		41,086
Investments in securities	1,105	39,910
Level 2 | Government Securities Money Market
Fair Value Measurements
Cash and cash equivalents	$ 90,866	$ 46,095