Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	$ 107,801
Unrealized Gains	292
Unrealized Losses	(668)
Fair Value	107,425
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	10,068
Unrealized Gains	77
Unrealized Losses	(209)
Fair Value	9,936
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	41,350	44,525
Unrealized Gains	200	0
Unrealized Losses	0	(44)
Fair Value	41,550	44,481
Corporate bonds
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	33,266	9,285
Unrealized Gains	6	1
Unrealized Losses	(236)	(24)
Fair Value	33,036	9,262
Government securities - U.S.
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	13,756	19,080
Unrealized Gains	0	0
Unrealized Losses	(200)	(30)
Fair Value	13,556	19,050
Other fixed-income securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Adjusted Cost	9,361	4,259
Unrealized Gains	9	0
Unrealized Losses	(23)	(9)
Fair Value	$ 9,347	$ 4,250

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