Fair Value Measurements - Summary of Asset and Liability Fair Values (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Equity securities (Note 7)	$ 32,153	$ 46,422
Restricted cash	4,020	0
Deferred compensation investments (life insurance cash surrender value (Note 3(e))	7,410	6,274
Total Assets	266,959	195,605
Liabilities:
Deferred compensation liability (Note 9(f))	7,433	6,167
Total Liabilities	7,433	6,167
Bank CDs [Member]
Assets:
Available-for-sale	6,584	86
Mutual Funds [Member]
Assets:
Available-for-sale	30	78
Money market funds [Member]
Assets:
Available-for-sale	72,384	142,745
US Government Debt Securities [Member]
Assets:
Available-for-sale	104,535
Corporate Debt Securities [Member]
Assets:
Available-for-sale	39,843
Level 1 [Member]
Assets:
Equity securities (Note 7)	32,153	46,422
Restricted cash	4,020
Deferred compensation investments (life insurance cash surrender value (Note 3(e))	0	0
Total Assets	168,333	46,422
Liabilities:
Deferred compensation liability (Note 9(f))	0	0
Total Liabilities	0	0
Level 1 [Member] \| Bank CDs [Member]
Assets:
Available-for-sale	0	0
Level 1 [Member] \| Mutual Funds [Member]
Assets:
Available-for-sale	0	0
Level 1 [Member] \| Money market funds [Member]
Assets:
Available-for-sale	72,384	0
Level 1 [Member] \| US Government Debt Securities [Member]
Assets:
Available-for-sale	59,776
Level 1 [Member] \| Corporate Debt Securities [Member]
Assets:
Available-for-sale	0
Level 2 [Member]
Assets:
Equity securities (Note 7)	0	0
Restricted cash	0
Deferred compensation investments (life insurance cash surrender value (Note 3(e))	7,410	6,274
Total Assets	98,626	149,183
Liabilities:
Deferred compensation liability (Note 9(f))	7,433	6,167
Total Liabilities	7,433	6,167
Level 2 [Member] \| Bank CDs [Member]
Assets:
Available-for-sale	6,584	86
Level 2 [Member] \| Mutual Funds [Member]
Assets:
Available-for-sale	30	78
Level 2 [Member] \| Money market funds [Member]
Assets:
Available-for-sale	0	142,745
Level 2 [Member] \| US Government Debt Securities [Member]
Assets:
Available-for-sale	44,759
Level 2 [Member] \| Corporate Debt Securities [Member]
Assets:
Available-for-sale	39,843
Level 3 [Member]
Assets:
Equity securities (Note 7)	0	0
Restricted cash	0
Deferred compensation investments (life insurance cash surrender value (Note 3(e))	0	0
Total Assets	0	0
Liabilities:
Deferred compensation liability (Note 9(f))	0	0
Total Liabilities	0	0
Level 3 [Member] \| Bank CDs [Member]
Assets:
Available-for-sale	0	0
Level 3 [Member] \| Mutual Funds [Member]
Assets:
Available-for-sale	0	0
Level 3 [Member] \| Money market funds [Member]
Assets:
Available-for-sale	0	$ 0
Level 3 [Member] \| US Government Debt Securities [Member]
Assets:
Available-for-sale	0
Level 3 [Member] \| Corporate Debt Securities [Member]
Assets:
Available-for-sale	$ 0

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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