Fair Value Disclosures - Summary of valuation of financial instruments measured at fair value on recurring basis (Details) - Fair value on a recurring basis - Total - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Available-for-sale securities - equity securities	$ 270	$ 264
Investments measured at net asset value - trading securities	23,463	18,367
Quoted prices in active markets for identical assets (Level 1)
Assets:
Available-for-sale securities - equity securities	270	264
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