Segment information - Revenues Generated in Domestic and Foreign Country (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue	$ 525,664	$ 501,569	$ 485,452	$ 443,039	$ 525,764	$ 553,210	$ 489,437	$ 438,229	$ 1,955,724	$ 2,006,640	$ 2,033,885
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue									1,825,186	1,889,144	1,925,865
International
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue									$ 130,538	$ 117,496	$ 108,020