Related Parties - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Oct. 26, 2015	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Related Party Transaction [Line Items]
Fees receivable		$ 468,352		$ 468,352		$ 537,442
Payments under tax receivable agreement				$ 25,454	$ 23,687
Amended and Restated Tax Receivable Agreement [Member]
Related Party Transaction [Line Items]
Amount of cash savings in income or franchise tax that would be realized as a result of increases in tax basis and certain other tax benefits related to the amended and restated tax receivable agreement	45.00%
Cash tax saving that may arise from tax benefits attributable to payments under the amended and restated tax receivable agreement	85.00%
Tax agreement termination year				2033
Sponsored Funds [Member]
Related Party Transaction [Line Items]
Revenue		$ 131,885	$ 141,365	$ 386,413	$ 438,733
Type of Revenue [Extensible List]		us-gaap:AssetManagement1Member	us-gaap:AssetManagement1Member	us-gaap:AssetManagement1Member	us-gaap:AssetManagement1Member
Fees receivable		$ 40,402		$ 40,402		54,561
LTBP Trust [Member]
Related Party Transaction [Line Items]
Cumulative liability relating to obligations under Amended and Restated Tax Receivable Agreement		$ 221,890		221,890		$ 247,344
Payments under tax receivable agreement				$ 25,454