Investments and Fair Value Measurements - Investment Category and Length (Details)	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) subsidiary	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($) subsidiary company	Dec. 31, 2018 USD ($) company	Sep. 30, 2018 subsidiary
Summary of fair value and gross unrealized losses related to available-for-sale securities [Abstract]
Fair value, in loss position less than twelve months	$ 4,519,000		$ 4,519,000	$ 9,892,000
Gross unrealized loss, in loss position less than twelve months	(13,000)		(13,000)	(32,000)
Fair value, in loss position greater than twelve months				12,935,000
Gross unrealized loss, in loss position greater than twelve months				(52,000)
Fair value, total in loss position	4,519,000		4,519,000	22,827,000
Gross unrealized loss, total in loss position	(13,000)		(13,000)	(84,000)
Minority Investments
Investments, equity method	$ 6,094,000		$ 6,094,000	$ 3,958,000
Number of consolidated subsidiaries | subsidiary	1		1		1
Number of equity method investments | company			5	7
Impairment charge	$ 0	$ 1,100,000	$ 1,100,000	$ 0
Xilingol Tongli Germanium Co. Ltd Investment
Minority Investments
Ownership (as a percent)	25.00%		25.00%
Xilingol Tongli Germanium Co. Ltd Investment | Xilingol Tongli Germanium Co. Ltd Investment
Minority Investments
Investments, equity method	$ 0		$ 0	1,700,000
Ownership (as a percent)	25.00%		25.00%
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				717,000
Gross unrealized loss, in loss position less than twelve months				(3,000)
Fair value, in loss position greater than twelve months				3,746,000
Gross unrealized loss, in loss position greater than twelve months				(24,000)
Fair value, total in loss position				4,463,000
Gross unrealized loss, total in loss position				(27,000)
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	$ 4,519,000		$ 4,519,000	9,175,000
Gross unrealized loss, in loss position less than twelve months	(13,000)		(13,000)	(29,000)
Fair value, in loss position greater than twelve months				9,189,000
Gross unrealized loss, in loss position greater than twelve months				(28,000)
Fair value, total in loss position	4,519,000		4,519,000	18,364,000
Gross unrealized loss, total in loss position	(13,000)		(13,000)	(57,000)
Other assets
Minority Investments
Investments, equity method	$ 6,100,000		$ 6,100,000	$ 8,400,000