Fair Value Measurements - Assets (Details) - USD ($) $ in Thousands	Dec. 31, 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	34,036	$ 28,369
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	33,250	22,909
Recurring | Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	33,250	22,909
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	786	5,460
Recurring | Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	778	5,450
Recurring | Level 2 | Mortgage-CMO debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total short-term investments	$ 8	$ 10