Segment information - Revenues Generated in Domestic and Foreign Country (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue	$ 469,934	$ 507,640	$ 492,869	$ 437,229	$ 525,664	$ 501,569	$ 485,452	$ 443,039	$ 1,907,672	$ 1,955,724	$ 2,006,640
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue									1,787,843	1,825,186	1,889,144
International
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue									$ 119,829	$ 130,538	$ 117,496