Investments (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Investments in privately-held companies [Abstract]
Equity investments without readily determinable fair value	$ 35,000,000	$ 35,000,000	$ 36,100,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment loss on equity investments	0	10,300,000
Unrealized gain on equity investments remeasured at fair value		1,200,000
Fair Value, Measurements, Nonrecurring | Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Re-measured or impaired equity investments	$ 27,000,000	$ 27,000,000