Summary of Significant Accounting Policies (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash and cash equivalents
Cash deposits	$ 16,628,000	$ 38,942,000
Money market funds	4,822,000	3,460,000
Commercial paper	1,000,000	2,646,000
Total cash and cash equivalents	22,450,000	45,048,000	$ 58,753,000	$ 30,136,000
Short-term investments
Commercial paper (held-to-maturity)	5,178,000	6,778,000
Bonds (held-to-maturity)	14,101,000	16,988,000
Allowance for credit losses	(21,000)	0
Total short-term investments	19,258,000	23,766,000
Long-term investments
Mutual funds (available for sale)	563,000	1,105,000
Bonds (held-to-maturity)	41,619,000	27,845,000
Allowance for credit losses	(52,000)	0
Equity securities (available for sale)	4,077,000	2,822,000
Total long-term investments	46,207,000	31,772,000
Total cash, cash equivalents and short and long-term investments	$ 87,915,000	$ 100,586,000