Fair Value Disclosures - Summary of valuation of financial instruments measured at fair value on recurring basis (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets:
Available-for-sale securities - equity securities	$ 264	$ 259
Investments measured at net asset value - trading securities	18,367	16,081
Fair value on a recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets:
Available-for-sale securities - equity securities	264	259
Fair value on a recurring basis | Significant other observable inputs (Level 2)
Assets:
Available-for-sale securities - equity securities
Fair value on a recurring basis | Significant unobservable inputs (Level 3)
Assets:
Available-for-sale securities - equity securities