Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Investments
Private common equity securities	$ 3,000		$ 3,000
Equity securities
Investments
Trading securities, change in unrealized holding gains (loss)	0	$ 2,900	0	$ (21,000)
Debt Securities
Investments
Trading securities, change in unrealized holding gains (loss)	$ 1,000	$ (17,700)	$ 2,400	$ (18,400)