Revenues and Business Information - Schedule of Geographical Distribution of Revenues and Assets (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	$ 207,723	$ 170,041	$ 647,546	$ 515,448
Total assets	823,019		823,019		$ 699,068
United States
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	172,168	131,306	544,407	420,412
Total assets	685,563		685,563		519,933
Europe
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	30,651	24,939	67,983	63,595
Total assets	43,787		43,787		83,834
Rest of World
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	4,904	$ 13,796	35,156	$ 31,441
Total assets	$ 93,669		$ 93,669		$ 95,301