Fair Value Measurements - Hierarchy (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial Assets:
Total financial assets	$ 94,393	$ 134,000
Money market deposits
Financial Assets:
Total financial assets	588	26,539
U.S. Treasury securities
Financial Assets:
Total financial assets	57,632	75,685
U.S. agency securities
Financial Assets:
Total financial assets	12,938	12,891
Municipal securities
Financial Assets:
Total financial assets	23,235	18,885
Level 1
Financial Assets:
Total financial assets	58,220	102,224
Level 1 | Money market deposits
Financial Assets:
Total financial assets	588	26,539
Level 1 | U.S. Treasury securities
Financial Assets:
Total financial assets	57,632	75,685
Level 2
Financial Assets:
Total financial assets	36,173	31,776
Level 2 | U.S. agency securities
Financial Assets:
Total financial assets	12,938	12,891
Level 2 | Municipal securities
Financial Assets:
Total financial assets	$ 23,235	$ 18,885