Fair Value Measurements - Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
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	40,590	$ 34,036
Level 1 | Available-for-sale securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	40,600	34,000
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	39,556	33,250
Recurring | Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	39,556	33,250
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	1,034	786
Recurring | Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	1,026	778
Recurring | Level 2 | Mortgage-CMO debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	$ 8	$ 8