Revenue from Contracts with Customers - Disaggregation of Revenue by Performance Obligation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Total Revenue	$ 358,025	$ 340,340	$ 321,479	$ 307,050	$ 307,216	$ 308,616	$ 255,993	$ 263,852	$ 1,326,894	$ 1,135,677	$ 1,102,924
Asset Management
Disaggregation of Revenue [Line Items]
Total Revenue									907,605	773,418
Administrative Services
Disaggregation of Revenue [Line Items]
Total Revenue									245,887	199,269
Distribution
Disaggregation of Revenue [Line Items]
Total Revenue									151,106	146,595
Other
Disaggregation of Revenue [Line Items]
Total Revenue									$ 22,296	$ 16,395