Fair Value Measurements - Summary of Asset and Liability Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Equity securities (Note 7)	$ 20,497	$ 31,047
Key employee life insurance, cash surrender value - Note 3(e))	3,073	3,547
Total Assets	174,805	222,212
Liabilities:
Deferred executive compensation liability (Note 8(f))	7,447	8,746
Total Liabilities	7,447	8,746
Government-related debt securities
Assets:
Available-for-sale	50,437	62,626
Corporate debt securities
Assets:
Available-for-sale	33,744	58,248
Money market funds
Assets:
Available-for-sale	54,282	54,199
Bank CDs
Assets:
Available-for-sale	8,120	7,376
Mutual funds
Assets:
Available-for-sale	4,652	5,169
Level 1
Assets:
Equity securities (Note 7)	20,497	31,047
Key employee life insurance, cash surrender value - Note 3(e))	0	0
Total Assets	114,833	138,040
Liabilities:
Deferred executive compensation liability (Note 8(f))	0	0
Total Liabilities	0	0
Level 1 | Government-related debt securities
Assets:
Available-for-sale	35,411	47,636
Level 1 | Corporate debt securities
Assets:
Available-for-sale	0	0
Level 1 | Money market funds
Assets:
Available-for-sale	54,282	54,199
Level 1 | Bank CDs
Assets:
Available-for-sale	0	0
Level 1 | Mutual funds
Assets:
Available-for-sale	4,643	5,158
Level 2
Assets:
Equity securities (Note 7)	0	0
Key employee life insurance, cash surrender value - Note 3(e))	3,073	3,547
Total Assets	59,972	84,172
Liabilities:
Deferred executive compensation liability (Note 8(f))	7,447	8,746
Total Liabilities	7,447	8,746
Level 2 | Government-related debt securities
Assets:
Available-for-sale	15,026	14,990
Level 2 | Corporate debt securities
Assets:
Available-for-sale	33,744	58,248
Level 2 | Money market funds
Assets:
Available-for-sale	0	0
Level 2 | Bank CDs
Assets:
Available-for-sale	8,120	7,376
Level 2 | Mutual funds
Assets:
Available-for-sale	9	11
Level 3
Assets:
Equity securities (Note 7)	0	0
Key employee life insurance, cash surrender value - Note 3(e))	0	0
Total Assets	0	0
Liabilities:
Deferred executive compensation liability (Note 8(f))	0	0
Total Liabilities	0	0
Level 3 | Government-related debt securities
Assets:
Available-for-sale	0	0
Level 3 | Corporate debt securities
Assets:
Available-for-sale	0	0
Level 3 | Money market funds
Assets:
Available-for-sale	0	0
Level 3 | Bank CDs
Assets:
Available-for-sale	0	0
Level 3 | Mutual funds
Assets:
Available-for-sale	$ 0	$ 0