Fair Value Measurements - Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 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
Total short-term investments	41,033	$ 28,369
Level 1 | Available-for-sale securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	41,000	28,400
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	38,691	22,909
Recurring | Level 1 | Available-for-sale equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Available-for-sale equity securities	38,691	22,909
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	2,342	5,460
Recurring | Level 2 | Available-for-sale equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Available-for-sale equity securities	2,333	5,450
Recurring | Level 2 | Mortgage-CMO debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Mortgage-CMO debt securities	$ 9	$ 10