Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Interest paid	$ 129,559	$ 143,990	$ 144,973
Income taxes paid	13,692	14,062	13,379
Changes in accounts payable related to purchases of revenue earning equipment	80,781	(142,256)	28,134
Operating and revenue earning equipment acquired under capital leases	$ 7,057	$ 1,230	$ 5,959