Fair Value Measurements - Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
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
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short term investments	28,400	$ 31,100
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short term investments	22,909
Recurring | Level 1 | Available-for-sale equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short term investments	22,909
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short term investments	5,460
Recurring | Level 2 | Available-for-sale equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short term investments	5,450
Recurring | Level 2 | Mortgage-CMO debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short term investments	$ 10