Fair Value Disclosures - Summary of valuation of financial instruments measured at fair value on recurring basis (Details) - Fair value on a recurring basis - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Available-for-sale securities - equity securities	$ 211	$ 270
Investments measured at net asset value - trading securities	22,815	23,463
Quoted prices in active markets for identical assets (Level 1)
Assets:
Available-for-sale securities - equity securities	211	270
Significant other observable inputs (Level 2)
Assets:
Available-for-sale securities - equity securities	0	0
Significant unobservable inputs (Level 3)
Assets:
Available-for-sale securities - equity securities	$ 0	$ 0