Fair Value Measurements - Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Amount of transfers of financial assets between Level 1 and Level 2 measures	$ 0		$ 0
Total short-term investments	43,262	$ 28,369
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short term investments	43,300	28,400
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short term investments		22,909
Total short-term investments	39,327
Recurring | Level 1 | Available-for-sale equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Available-for-sale securities	39,327	22,909
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short term investments		5,460
Total short-term investments	3,935
Recurring | Level 2 | Available-for-sale equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Available-for-sale securities	3,926	5,450
Recurring | Level 2 | Mortgage-CMO debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short term investments	$ 9	$ 10