Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 102,877	$ 84,583	$ 84,431	$ 69,744	$ 76,932	$ 68,138	$ 64,292	$ 64,686	$ 341,635	$ 274,048	$ 252,179
Management and advisory fees
Disaggregation of Revenue [Line Items]
Revenues									289,444	244,920	217,773
Specialized funds
Disaggregation of Revenue [Line Items]
Revenues									148,023	111,803	93,056
Customized separate accounts
Disaggregation of Revenue [Line Items]
Revenues									93,963	90,750	85,245
Advisory
Disaggregation of Revenue [Line Items]
Revenues									26,439	24,160	24,130
Reporting and other
Disaggregation of Revenue [Line Items]
Revenues									11,134	9,102	8,805
Distribution management
Disaggregation of Revenue [Line Items]
Revenues									6,701	4,920	4,525
Fund reimbursement revenue
Disaggregation of Revenue [Line Items]
Revenues									3,184	4,185	2,012
Incentive fees
Disaggregation of Revenue [Line Items]
Revenues									52,191	29,128	34,406
Specialized funds
Disaggregation of Revenue [Line Items]
Revenues									13,241	6,680	8,939
Customized separate accounts
Disaggregation of Revenue [Line Items]
Revenues									17,893	14,757	8,719
Specialized funds
Disaggregation of Revenue [Line Items]
Revenues									31,134	21,437	17,658
Specialized funds | Primary Beneficiary
Disaggregation of Revenue [Line Items]
Revenues									$ 21,057	$ 7,691	$ 16,748