Investments and Fair Value Measurements - Investment Category and Length (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) company	Mar. 31, 2017 USD ($) entity	Dec. 31, 2017 USD ($)	Mar. 31, 2017 company	Mar. 31, 2017 USD ($)
Summary of fair value and gross unrealized losses related to available-for-sale securities [Abstract]
Fair value, in loss position less than twelve months	$ 18,088,000		$ 29,527,000
Gross unrealized loss, in loss position less than twelve months	(142,000)		(85,000)
Fair value, in loss position greater than twelve months	9,598,000		2,342,000
Gross unrealized loss, in loss position greater than twelve months	(68,000)		(8,000)
Fair value, total in loss position	27,686,000		31,869,000
Gross unrealized loss, total in loss position	(210,000)		(93,000)
Minority Investments
Investments in privately-held companies	$ 5,453,000		5,578,000
Equity method investments | company	7
Impairment charge	$ 0
One Gallium Company
Minority Investments
Investments in privately-held companies					$ 0
Number of gallium companies with impairment charge		1		1
Impairment charge		$ 313,000
Certificates of Deposit [Member]
Summary of fair value and gross unrealized losses related to available-for-sale securities [Abstract]
Fair value, in loss position less than twelve months	4,935,000		3,994,000
Gross unrealized loss, in loss position less than twelve months	(35,000)		(16,000)
Fair value, in loss position greater than twelve months	2,344,000		2,342,000
Gross unrealized loss, in loss position greater than twelve months	(6,000)		(8,000)
Fair value, total in loss position	7,279,000		6,336,000
Gross unrealized loss, total in loss position	(41,000)		(24,000)
Corporate Bonds [Member]
Summary of fair value and gross unrealized losses related to available-for-sale securities [Abstract]
Fair value, in loss position less than twelve months	13,153,000		25,533,000
Gross unrealized loss, in loss position less than twelve months	(107,000)		(69,000)
Fair value, in loss position greater than twelve months	7,254,000
Gross unrealized loss, in loss position greater than twelve months	(62,000)
Fair value, total in loss position	20,407,000		25,533,000
Gross unrealized loss, total in loss position	(169,000)		(69,000)
Other assets
Minority Investments
Investments in privately-held companies	$ 9,700,000		$ 9,800,000