Revenues - Financial Statement Impact of Adopting ASC 606 (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 129,028	$ 79,776
Distribution and other asset-based expenses	22,291	15,323
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Distribution and other asset-based expenses	21,055
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Distribution and other asset-based expenses	1,236
Distribution and service fees
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	12,607	$ 10,783
Distribution and service fees | Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	11,371
Distribution and service fees | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 1,236