Fair Value Measurements - Fair Value Hierarchy (Details)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) item
Fair Value, Inputs, Level 3
Assets
Total assets	$ 0	$ 0
Liabilities
Total liabilities	0	0
Fair Value, Measurements, Recurring
Assets
Total assets	324,967,000	323,273,000
Liabilities
Total liabilities	7,302,000	5,232,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1
Assets
Total assets	4,212,000	5,169,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	320,755,000	318,104,000
Liabilities
Total liabilities	7,302,000	5,232,000
Money market funds \| Fair Value, Measurements, Recurring
Assets
Total assets	4,212,000	5,169,000
Money market funds \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1
Assets
Total assets	4,212,000	5,169,000
U.S. Government agency bonds \| Fair Value, Measurements, Recurring
Assets
Total assets	123,271,000	206,924,000
U.S. Government agency bonds \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	123,271,000	$ 206,924,000
U.S. Government bonds \| Fair Value, Measurements, Recurring
Assets
Total assets	76,525,000
U.S. Government bonds \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	76,525,000
Bank certificates of deposit
Assets
Number of certificates of deposit \| item		1
Bank certificates of deposit \| Fair Value, Measurements, Recurring
Assets
Cash equivalents		$ 5,000,000
Total assets	12,492,000	25,708,000
Bank certificates of deposit \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	12,492,000	25,708,000
Commercial paper. \| Fair Value, Measurements, Recurring
Assets
Total assets	2,997,000	1,500,000
Commercial paper. \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	2,997,000	1,500,000
Corporate notes \| Fair Value, Measurements, Recurring
Assets
Total assets	55,032,000	55,173,000
Corporate notes \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	55,032,000	55,173,000
Asset-backed securities \| Fair Value, Measurements, Recurring
Assets
Total assets	23,774,000	13,495,000
Asset-backed securities \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	23,774,000	13,495,000
Municipal bonds \| Fair Value, Measurements, Recurring
Assets
Total assets	19,252,000	9,972,000
Municipal bonds \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	19,252,000	9,972,000
Investments held for deferred compensation plans \| Fair Value, Measurements, Recurring
Assets
Total assets	7,412,000	5,331,000
Liabilities
Total liabilities	7,302,000	5,232,000
Investments held for deferred compensation plans \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets
Total assets	7,412,000	5,331,000
Liabilities
Total liabilities	$ 7,302,000	$ 5,232,000