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Non-Marketable Investments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]
Carrying value of the Company's non-marketable investments	$ 2,500,000	$ 1,900,000
Book value of investment			$ 400,000
Gain (losses) from non-marketable investments	600,000	(500,000)	(800,000)
Distributions received from funds	$ 0	$ 400,000	$ 0
Limited Partnerships Investments [Member] | Minimum [Member]
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]
Ownership interest of Company	5.00%

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Carrying amount of investments in equity method investees and the carrying amount, after adjustment, of cost-method investments.

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Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments.

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The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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Percentage of the Variable Interest Entity's (VIE) voting interest owned by (or beneficial interest in) the reporting entity (directly or indirectly).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
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