Income Taxes (Unrecognized Tax Benefits) (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued [Abstract]
Reserve for unrecognized tax benefits, including interest and penalties	$ 40.9	$ 40.6	$ 43.6
Amounts that would affect the effective tax rate if recognized	40.9	40.6	43.6
Federal tax effect for unrecognized tax benefits	8.3
Accrual of interest and penalties related to uncertain tax positions	5.5	5.8	6.7
Tax expense related to an increase in liabilities for interest and penalties	2.0	$ 2.4	$ 2.8
Reduction of unrecognized tax benefits due to lapse of statute of limitations and settlements over the next twelve months	$ 7.5
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 2012 and 2013 federal income tax returns in the second quarter of 2015. The statute of limitations for the Company’s federal income tax returns for tax years 2013 and prior expired on September 15, 2017. The statute of limitations for the Company’s U.S. federal income tax return for 2014 will expire on September 15, 2018, unless otherwise extended. The IRS is currently conducting an examination of the Company’s consolidated returns for tax years 2014 and 2015. The Company’s state income tax returns remain subject to examination by various state authorities for tax years ranging from 2006 through 2016.
Open tax year	2015