Revenues - Financial Statement Impact of Adopting ASC 606 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 552,235	$ 425,607	$ 322,554
Distribution and other asset-based expenses	92,441	71,987	69,049
Balance Under Prior ASC 605 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Distribution and other asset-based expenses	86,465
Effect of Change Higher/(Lower) | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Distribution and other asset-based expenses	5,976
Distribution and service fees
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	50,715	$ 44,322	$ 48,250
Distribution and service fees | Balance Under Prior ASC 605 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	44,739
Distribution and service fees | Effect of Change Higher/(Lower) | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 5,976