Investments (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Cash and cash equivalents:
Total cash and cash equivalents	$ 24,099,000	$ 45,048,000
Short-term investments:
Total short-term Investments	6,573,000	23,766,000
Allowance for credit losses	(16,000)	0
Long-term investments:
Total long-term investments	56,872,000	31,772,000
Allowance for credit losses	(66,000)	0
Total cash, cash equivalents and short and long-term investments	87,544,000	100,586,000
Commercial Paper (Held-To-Maturity) [Member]
Short-term investments:
Total short-term Investments	4,709,000	6,778,000
Bonds (Held-To-Maturity) [Member]
Short-term investments:
Total short-term Investments	1,880,000	16,988,000
Mutual funds (available for sale) [Member]
Long-term investments:
Total long-term investments	462,000	1,105,000
Bonds [Held-To-Maturity] [Member]
Long-term investments:
Total long-term investments	54,353,000	27,845,000
Equity securities (available for sale] [Member]
Long-term investments:
Total long-term investments	2,123,000	2,822,000
Cash Deposits [Member]
Cash and cash equivalents:
Total cash and cash equivalents	21,488,000	38,942,000
Money Market Funds [Member]
Cash and cash equivalents:
Total cash and cash equivalents	1,861,000	3,460,000
Commercial Paper [Member]
Cash and cash equivalents:
Total cash and cash equivalents	$ 750,000	$ 2,646,000