Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Information [Abstract]
Non-cash investing and financing activities related to property acquired under capital leases	$ 11.5		$ 27.3
Capital lease and deferred purchase price related to acquisitions	3.3	$ 47.2
Accrued purchases of property and equipment	48.5	29.1	25.3
Interest payments, net of amounts capitalized and including interest on vehicle inventory financing	205.9	183.9	135.3
Income tax payments, net of income tax refunds	$ 127.0	$ 265.5	$ 278.8