Revenues - Financial Statement Impact of Adopting ASC 606 (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 152,210	$ 123,675	$ 414,170	$ 297,583
Distribution and other asset-based expenses	25,386	20,552	71,398	51,639
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	23,281		65,963
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	2,105		5,435
Distribution and service fees
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	13,730	$ 11,482	39,886	$ 32,704
Distribution and service fees | Balance Under Prior ASC 605 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	11,625		34,451
Distribution and service fees | Effect of Change Higher/(Lower) | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 2,105		$ 5,435