Revenue from Contracts with Customers - Disaggregation of Revenue by Performance Obligation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Total Revenue	$ 360,712	$ 321,479	$ 719,895	$ 628,529
Asset Management
Disaggregation of Revenue [Line Items]
Total Revenue	245,126	220,669	485,787	431,868
Administrative Services
Disaggregation of Revenue [Line Items]
Total Revenue	83,733	57,968	155,932	112,103
Distribution
Disaggregation of Revenue [Line Items]
Total Revenue	25,579	37,196	64,513	73,442
Other
Disaggregation of Revenue [Line Items]
Total Revenue	$ 6,274	$ 5,646	$ 13,663	$ 11,116