Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Marketable Securities [Abstract]
Fair value of investments owned	$ 169,200		$ 169,200		$ 164,800
Trading securities with market value	158,400		158,400		154,800
Realized and unrealized net gain from trading securities	5,500	$ 7,500	11,500	$ 6,700
Investments classified as available-for-sale, fair value	10,800		10,800		$ 10,000
Gains (loss) recognized related to marketable securities classified as available-for-sale	500	(1,900)	800	(1,200)
Marketable securities sold at market value	$ 7,800	$ 566,300	7,800	566,300
Purchase of marketable securities			$ 0	$ 63,341