Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	$ 91,359
Unrealized Gains	411
Unrealized Losses	(1,180)
Fair Value	90,590
ASU 2016-01 Transition [Abstract]
Adjusted Cost		$ 77,592
Unrealized Gains		32
Unrealized Losses		(113)
Fair Value		77,511
Equity securities
Debt Securities, Trading, and Equity Securities, FV-NI [Abstract]
Adjusted Cost	10,101
Unrealized Gains	0
Unrealized Losses	(956)
Fair Value	9,145
ASU 2016-01 Transition [Abstract]
Adjusted Cost		443
Unrealized Gains		31
Unrealized Losses		(6)
Fair Value		468
Money market funds
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	52,733
Unrealized Gains	316
Unrealized Losses	(53)
Fair Value	52,996
ASU 2016-01 Transition [Abstract]
Adjusted Cost		44,525
Unrealized Gains		0
Unrealized Losses		(44)
Fair Value		44,481
Corporate bonds
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	6,411
Unrealized Gains	7
Unrealized Losses	(79)
Fair Value	6,339
ASU 2016-01 Transition [Abstract]
Adjusted Cost		9,285
Unrealized Gains		1
Unrealized Losses		(24)
Fair Value		9,262
Government securities - U.S.
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	12,714
Unrealized Gains	66
Unrealized Losses	(79)
Fair Value	12,701
ASU 2016-01 Transition [Abstract]
Adjusted Cost		19,080
Unrealized Gains		0
Unrealized Losses		(30)
Fair Value		19,050
Other fixed-income securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	9,400
Unrealized Gains	22
Unrealized Losses	(13)
Fair Value	$ 9,409
ASU 2016-01 Transition [Abstract]
Adjusted Cost		4,259
Unrealized Gains		0
Unrealized Losses		(9)
Fair Value		$ 4,250

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