Fair Value Measurements - Fair Value by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
One Year or Less	$ 75,692	$ 27,041
Over One Year Through Five Years	126,452	40,505
Over Five Years	0	0
Total	202,144	67,546
U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
One Year or Less	15,072	6,075
Over One Year Through Five Years	36,871	15,265
Over Five Years	0	0
Total	51,943	21,340
Yankee bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
One Year or Less	0
Over One Year Through Five Years	3,058
Over Five Years	0
Total	3,058
Agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
One Year or Less	14,091
Over One Year Through Five Years	3,499
Over Five Years	0
Total	17,590
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
One Year or Less	0
Over One Year Through Five Years	12,411
Over Five Years	0
Total	12,411
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
One Year or Less	21,964	14,543
Over One Year Through Five Years	0	0
Over Five Years	0	0
Total	21,964	14,543
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
One Year or Less	16	560
Over One Year Through Five Years	26,588	13,978
Over Five Years	0	0
Total	26,604	14,538
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
One Year or Less	24,549	5,863
Over One Year Through Five Years	44,025	11,262
Over Five Years	0	0
Total	$ 68,574	$ 17,125