Fair Value Measurements - Summary of Asset and Liability Fair Values (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets:
Equity securities (Note 7)	$ 23,085	$ 31,047
Key employee life insurance, cash surrender value	3,484	3,547
Total Assets	158,870	222,212
Liabilities:
Deferred executive compensation liability (Note 8(f))	7,582	8,746
Total Liabilities	7,582	8,746
Government-related debt securities
Assets:
Available-for-sale	20,293	62,626
Corporate debt securities
Assets:
Available-for-sale	19,739	58,248
Money market funds
Assets:
Available-for-sale	83,987	54,199
Bank CDs
Assets:
Available-for-sale	4,161	7,376
Mutual funds
Assets:
Available-for-sale	4,121	5,169
Level 1
Assets:
Equity securities (Note 7)	23,085	31,047
Key employee life insurance, cash surrender value	0	0
Total Assets	131,477	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	20,293	47,636
Level 1 | Corporate debt securities
Assets:
Available-for-sale	0	0
Level 1 | Money market funds
Assets:
Available-for-sale	83,987	54,199
Level 1 | Bank CDs
Assets:
Available-for-sale	0	0
Level 1 | Mutual funds
Assets:
Available-for-sale	4,112	5,158
Level 2
Assets:
Equity securities (Note 7)	0	0
Key employee life insurance, cash surrender value	3,484	3,547
Total Assets	27,393	84,172
Liabilities:
Deferred executive compensation liability (Note 8(f))	7,582	8,746
Total Liabilities	7,582	8,746
Level 2 | Government-related debt securities
Assets:
Available-for-sale	0	14,990
Level 2 | Corporate debt securities
Assets:
Available-for-sale	19,739	58,248
Level 2 | Money market funds
Assets:
Available-for-sale	0	0
Level 2 | Bank CDs
Assets:
Available-for-sale	4,161	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	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