Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	$ 61,367	$ 91,359
Unrealized Gains	916	411
Unrealized Losses	(132)	(1,180)
Fair Value	62,151	90,590
Equity securities
Debt Securities, Trading, and Equity Securities, FV-NI [Abstract]
Adjusted Cost	4,166	10,101
Unrealized Gains	0	0
Unrealized Losses	(128)	(956)
Fair Value	4,038	9,145
Money market funds
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	46,314	52,733
Unrealized Gains	719	316
Unrealized Losses	0	(53)
Fair Value	47,033	52,996
Corporate bonds
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	3,611	6,411
Unrealized Gains	54	7
Unrealized Losses	(1)	(79)
Fair Value	3,664	6,339
Government securities - U.S.
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	5,122	12,714
Unrealized Gains	114	66
Unrealized Losses	(2)	(79)
Fair Value	5,234	12,701
Other fixed-income securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	2,154	9,400
Unrealized Gains	29	22
Unrealized Losses	(1)	(13)
Fair Value	$ 2,182	$ 9,409

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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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