Investments and Fair Value Measurements - Investment Category and Length (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) company	Jun. 30, 2017 USD ($) company	Jun. 30, 2018 USD ($) company	Jun. 30, 2017 USD ($) company	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)
Summary of fair value and gross unrealized losses related to available-for-sale securities [Abstract]
Fair value, in loss position less than twelve months	$ 16,064,000		$ 16,064,000		$ 29,527,000
Gross unrealized loss, in loss position less than twelve months	(121,000)		(121,000)		(85,000)
Fair value, in loss position greater than twelve months	8,433,000		8,433,000		2,342,000
Gross unrealized loss, in loss position greater than twelve months	(49,000)		(49,000)		(8,000)
Fair value, total in loss position	24,497,000		24,497,000		31,869,000
Gross unrealized loss, total in loss position	(170,000)		(170,000)		(93,000)
Minority Investments
Investments, equity method	$ 5,373,000		$ 5,373,000		5,578,000
Equity method investments | company	7	7	7	7
Impairment charge	$ 0	$ 0	$ 0
One Gallium Company
Minority Investments
Investments, equity method						$ 0
Number of gallium companies with impairment charge | company		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	4,931,000		4,931,000		3,994,000
Gross unrealized loss, in loss position less than twelve months	(39,000)		(39,000)		(16,000)
Fair value, in loss position greater than twelve months	1,196,000		1,196,000		2,342,000
Gross unrealized loss, in loss position greater than twelve months	(4,000)		(4,000)		(8,000)
Fair value, total in loss position	6,127,000		6,127,000		6,336,000
Gross unrealized loss, total in loss position	(43,000)		(43,000)		(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	11,133,000		11,133,000		25,533,000
Gross unrealized loss, in loss position less than twelve months	(82,000)		(82,000)		(69,000)
Fair value, in loss position greater than twelve months	7,237,000		7,237,000
Gross unrealized loss, in loss position greater than twelve months	(45,000)		(45,000)
Fair value, total in loss position	18,370,000		18,370,000		25,533,000
Gross unrealized loss, total in loss position	(127,000)		(127,000)		(69,000)
Other assets
Minority Investments
Investments, equity method	$ 9,700,000		$ 9,700,000		$ 9,800,000