Revenue from Contracts with Customers - Disaggregation of Revenue by Performance Obligation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total Revenue	$ 307,216	$ 308,616	$ 255,993	$ 263,852	$ 278,312	$ 278,315	$ 272,796	$ 273,501	$ 1,135,677	$ 1,102,924	$ 1,143,371
Asset Management
Disaggregation of Revenue [Line Items]
Total Revenue									773,418
Administrative Services
Disaggregation of Revenue [Line Items]
Total Revenue									199,269
Distribution
Disaggregation of Revenue [Line Items]
Total Revenue									146,595
Other
Disaggregation of Revenue [Line Items]
Total Revenue									$ 16,395