Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	$ 308,616	$ 278,315	$ 828,461	$ 824,612
Other Long-Term Assets | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment, shorter amortization period					$ 8,100
Cumulative-effect adjustment, revenue					(8,000)
Retained Earnings | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment, shorter amortization period					(8,100)
Cumulative-effect adjustment, revenue					$ 8,000
Other Service Fees, net—Affiliates
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	39,046	44,539	119,105	134,911
Other Service Fees, net—Affiliates | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	400		1,200
Investment Advisory Fees, net—Affiliates
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	159,527	149,412	425,288	442,536
Investment Advisory Fees, net—Affiliates | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	(8,700)		(25,600)
Investment Advisory Fees, net—Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	57,509	$ 35,474	134,141	$ 102,989
Investment Advisory Fees, net—Other | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	$ 1,500		$ 4,800