Fair Value Measurements - Hierarchy (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Fair Value Measurements
Fair value, transfers between Level I and Level II assets	$ 0
Recurring
Financial Assets:
Total financial assets	208,683	$ 134,000
Recurring | Money market deposits
Financial Assets:
Total financial assets	630	26,539
Recurring | U.S. Treasury securities
Financial Assets:
Total financial assets	159,097	75,685
Recurring | U.S. agency securities
Financial Assets:
Total financial assets	12,977	12,891
Recurring | Municipal securities
Financial Assets:
Total financial assets	35,979	18,885
Recurring | Level 1
Financial Assets:
Total financial assets	159,727	102,224
Recurring | Level 1 | Money market deposits
Financial Assets:
Total financial assets	630	26,539
Recurring | Level 1 | U.S. Treasury securities
Financial Assets:
Total financial assets	159,097	75,685
Recurring | Level 2
Financial Assets:
Total financial assets	48,956	31,776
Recurring | Level 2 | U.S. agency securities
Financial Assets:
Total financial assets	12,977	12,891
Recurring | Level 2 | Municipal securities
Financial Assets:
Total financial assets	$ 35,979	$ 18,885