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Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Equity [Line Items]
Fair value of investments owned	$ 152,500		$ 152,500		$ 208,200
Marketable securities	152,485		152,485		208,176
Realized and unrealized net gains (losses) on marketable securities	(4,100)		12,400
Earn-out share payment			85,135	$ 76,969
Trading securities with fair value					198,300
Realized and unrealized net gain from trading securities		$ 12,800		24,300
Investments classified as available-for-sale, fair value					$ 9,900
Gains (loss) recognized related to marketable securities classified as available-for-sale		500		1,200
Marketable securities sold at fair value			153,251	7,849
Purchase of marketable securities			$ 0	$ 0
NASDAQ [Member] | Other Income (Loss) [Member]
Equity [Line Items]
Earn-out share payment	$ 85,100	$ 77,000

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Equity [Line Items]

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of unrealized and realized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Value of the investment at close of period. For schedules of investments that are categorized, the value would be aggregated by category. For investment in and advances to affiliates, if operations of any controlled companies are different in character from those of the company, group such affiliates within divisions and by type of activities.

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Amount of cash outflow for purchase of marketable security.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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