Fair Value Disclosures - Financial instruments measured at fair value on recurring basis (Details) - Fair value on a recurring basis - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Available-for-sale securities - equity securities	$ 132	$ 237
Investments measured at net asset value - trading securities	32,039	28,476
Level 1
Assets:
Available-for-sale securities - equity securities	132	237
Level 2
Assets:
Available-for-sale securities - equity securities	0	0
Level 3
Assets:
Available-for-sale securities - equity securities	$ 0	$ 0