Non-Marketable Investments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]
Carrying value of the Company's non-marketable investments	$ 1,900,000	$ 2,800,000
Book value of investment		400,000
Losses from non-marketable investments	(500,000)	(800,000)
Distributions received from funds	$ 400,000	$ 0	$ 100,000
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%