Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Jun. 28, 2013	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Equity [Line Items]
Fair value of investments owned		$ 259,400		$ 259,400		$ 164,800
Trading securities with market value		248,200		248,200		154,800
Realized and unrealized net gain from trading securities		12,800	$ 6,100	24,300	$ 12,800
Cumulative realized gain on marketable securities				1,222	11,792
Investments classified as available-for-sale, fair value		11,300		11,300		$ 10,000
Gains (loss) recognized related to marketable securities classified as available-for-sale		500	500	1,200	(700)
Marketable securities sold at market value		7,800	618,100	7,800	618,100
Purchase of marketable securities				$ 0	$ 68,396
NASDAQ [Member]
Equity [Line Items]
Period for earn-out receivable under common stock transaction	10 years
NASDAQ [Member] | Maximum [Member]
Equity [Line Items]
Remaining earn-out receivable under common stock transaction	9,922,470
NASDAQ [Member] | Minimum [Member]
Equity [Line Items]
Gross revenue on earn-out receivable per year under common stock transaction	$ 25,000
NASDAQ [Member] | Other Income (Loss) [Member]
Equity [Line Items]
Cumulative realized gain on marketable securities		$ 77,000	$ 67,000