Fair Value Measurements - Summary of Asset and Liability Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Equity securities (Note 7)	$ 33,142	$ 46,422
Equity forward-sale contract (Note 7)	793
Deferred compensation investments (life insurance cash surrender value (Note 3(e))	7,270	6,274
Restricted cash	4,000	0
Total Assets	302,360	195,605
Liabilities:
Deferred executive compensation liability (Note 9(f))	7,223	6,167
Total Liabilities	7,223	6,167
Bank CDs [Member]
Assets:
Available-for-sale	87	86
Money market funds [Member]
Assets:
Available-for-sale	257,003	142,745
Mutual Funds [Member]
Assets:
Available-for-sale	65	78
Level 1 [Member]
Assets:
Equity securities (Note 7)	33,142	46,422
Equity forward-sale contract (Note 7)	793
Deferred compensation investments (life insurance cash surrender value (Note 3(e))	0	0
Restricted cash	4,000
Total Assets	294,938	46,422
Liabilities:
Deferred executive 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] \| Money market funds [Member]
Assets:
Available-for-sale	257,003	0
Level 1 [Member] \| Mutual Funds [Member]
Assets:
Available-for-sale	0	0
Level 2 [Member]
Assets:
Equity securities (Note 7)	0	0
Equity forward-sale contract (Note 7)	0
Deferred compensation investments (life insurance cash surrender value (Note 3(e))	7,270	6,274
Restricted cash	0
Total Assets	7,422	149,183
Liabilities:
Deferred executive compensation liability (Note 9(f))	7,223	6,167
Total Liabilities	7,223	6,167
Level 2 [Member] \| Bank CDs [Member]
Assets:
Available-for-sale	87	86
Level 2 [Member] \| Money market funds [Member]
Assets:
Available-for-sale	0	142,745
Level 2 [Member] \| Mutual Funds [Member]
Assets:
Available-for-sale	65	78
Level 3 [Member]
Assets:
Equity securities (Note 7)	0	0
Equity forward-sale contract (Note 7)	0
Deferred compensation investments (life insurance cash surrender value (Note 3(e))	0	0
Restricted cash	0
Total Assets	0	0
Liabilities:
Deferred executive 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] \| Money market funds [Member]
Assets:
Available-for-sale	0	0
Level 3 [Member] \| Mutual Funds [Member]
Assets:
Available-for-sale	$ 0	$ 0

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