Revenue from Contracts with Customers - Disaggregation of Revenue by Performance Obligation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total Revenue	$ 340,340	$ 308,616	$ 968,869	$ 828,461
Asset Management
Disaggregation of Revenue [Line Items]
Total Revenue	232,106	217,036	663,974	559,429
Administrative Services
Disaggregation of Revenue [Line Items]
Total Revenue	64,216	49,855	176,319	147,248
Distribution
Disaggregation of Revenue [Line Items]
Total Revenue	38,501	36,226	111,942	111,206
Other
Disaggregation of Revenue [Line Items]
Total Revenue	$ 5,517	$ 5,499	$ 16,634	$ 10,578