Revenue (Disaggregation of Revenue by Geographical Areas) (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
Disaggregation of revenue
Revenues	$ 646,365	$ 629,735	$ 619,130	$ 562,684	$ 567,365	$ 528,643	$ 519,508	$ 501,221	$ 2,457,914	$ 2,116,737	$ 2,008,991
United States
Disaggregation of revenue
Revenues									2,145,098	1,837,525	1,760,214
Mexico
Disaggregation of revenue
Revenues									233,116	210,228	183,058
Other
Disaggregation of revenue
Revenues									$ 79,700	$ 68,984	$ 65,719