Summary of Significant Accounting Policies New Accountings Pronouncements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue	$ 381,111	$ 378,696	$ 351,497
Distribution and service fees	50,043	53,338	53,144
General and administrative	48,265	42,219	42,989
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		1,095
Investment advisory and administration fees
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue	341,226	336,743	311,859
Distribution and service fees		50,538	49,780
Distribution and service fees
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue	$ 29,090	30,747	28,825
General and administrative		42,219	42,989
Previously Reported | Investment advisory and administration fees
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue		346,832	319,667
Distribution and service fees		39,632	39,590
Previously Reported | Distribution and service fees
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue		20,156	19,396
General and administrative		52,623	51,558
Net Adjustments Due to New Revenue Standard | Investment advisory and administration fees | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue		(10,089)	(7,808)
Distribution and service fees		10,906	10,190
Net Adjustments Due to New Revenue Standard | Distribution and service fees | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue		10,591	9,429
General and administrative		$ (10,404)	$ (8,569)