Variable Interest Entities - Summary of Information about Variable Interests (Detail) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Carrying Amount - Assets
Total net carrying value		$ 28,172,000
Total net carrying value	$ 880,971,000	935,207,000
Maximum exposure to loss	8,112,000	36,284,000
Advisor Engine [Member]
Carrying Amount - Assets
Unsecured convertible notes receivable		2,126,000
Unsecured non-convertible note receivable		26,046,000
Preferred stock
Total net carrying value		28,172,000
Securrency Inc [Member]
Carrying Amount - Assets
Preferred stock	8,112,000	8,112,000
Variable Interest Entity, Not Primary Beneficiary [Member]
Carrying Amount - Assets
Total net carrying value	$ 8,112,000	$ 36,284,000