Recent Accounting Pronouncements (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	Jan. 01, 2019	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
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
Other Service Fees, net—Affiliates
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue									156,935	176,397	160,024
Investment Advisory Fees, net—Affiliates
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue									585,832	591,112	654,224
Investment Advisory Fees, net—Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue									187,586	$ 140,558	$ 112,601
Accounting Standards Update 2014-09 | Other Service Fees, net—Affiliates
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue									1,500
Accounting Standards Update 2014-09 | Investment Advisory Fees, net—Affiliates
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue									(34,300)
Accounting Standards Update 2014-09 | Investment Advisory Fees, net—Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue									$ 6,300
Accounting Standards Update 2014-09 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment													$ 8,100
Cumulative-effect adjustment													$ 8,000
Subsequent Event | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-use asset												$ 112,000
Lease liability												$ 134,000