Revenue from Contracts with Customers - Disaggregation of Revenue by Performance Obligation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total Revenue	$ 364,455	$ 340,340	$ 1,084,350	$ 968,869
Asset Management
Disaggregation of Revenue [Line Items]
Total Revenue	260,089	232,106	745,875	663,974
Administrative Services
Disaggregation of Revenue [Line Items]
Total Revenue	83,028	64,216	238,960	176,319
Distribution
Disaggregation of Revenue [Line Items]
Total Revenue	15,273	38,501	79,786	111,942
Other
Disaggregation of Revenue [Line Items]
Total Revenue	$ 6,065	$ 5,517	$ 19,729	$ 16,634