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Marketable Securities and Fair Value Measurements - Investments in Privately Held Companies (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Apr. 30, 2018	Dec. 31, 2017
Schedule of Fair Value Disclosure [Line Items]
Equity method investments	$ 45,913		$ 54,606
Equity securities without readily determinable fair value	$ 9,862		0
Fair Value, Measurements, Recurring
Schedule of Fair Value Disclosure [Line Items]
Short term notes receivable			4,476
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3
Schedule of Fair Value Disclosure [Line Items]
Short term notes receivable		$ 4,900	$ 4,476