Income Taxes (Unrecognized Tax Benefits) (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued [Abstract]
Reserve for unrecognized tax benefits, including interest and penalties	$ 38.9	$ 40.9	$ 40.6
Amounts that would affect the effective tax rate if recognized	38.9	40.9	40.6
Federal tax effect for unrecognized tax benefits	8.2
Accrual of interest and penalties related to uncertain tax positions	5.1	5.5	5.8
Tax expense related to an increase in liabilities for interest and penalties	2.3	$ 2.0	$ 2.4
Reduction of unrecognized tax benefits due to lapse of statute of limitations and settlements over the next twelve months	$ 8.1
Other information pertaining to income taxes	The Company’s United States federal income tax returns for tax years 2015 and beyond remain subject to examination by the Internal Revenue Service (“IRS”). The IRS concluded an examination of the O’Reilly consolidated 2014, 2015 and 2016 federal income tax returns in the third quarter of 2018. The Company’s state income tax returns remain subject to examination by various state authorities for tax years ranging from 2007 through 2017.
Open tax year	2015