Revenues - Financial Statement Impact of Adopting ASC 606 (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 132,932	$ 94,132	$ 261,960	$ 173,908
Distribution and other asset-based expenses	23,721	15,764	46,012	31,087
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	21,627		42,682
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	2,094		3,330
Distribution and service fees
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	13,549	$ 10,439	26,156	$ 21,222
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	11,455		22,826
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	$ 2,094		$ 3,330