Income Taxes (Unrecognized Tax Benefits) (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued [Abstract]
Reserve for unrecognized tax benefits, including interest and penalties	$ 43.6	$ 58.4	$ 58.6
Amounts that would affect the effective tax rate if recognized	43.6	58.4	58.6
Federal tax effect for unrecognized tax benefits	14.9
Accrual of interest and penalties related to uncertain tax positions	6.7	8.8	8.1
Tax expense related to an increase in liabilities for interest and penalties	2.8	$ 2.8	$ 2.1
Reduction of unrecognized tax benefits due to lapse of statute of limitations and settlements over the next twelve months	$ 10.2
Other information pertaining to income taxes	The Company's United States federal income tax returns for tax years 2014 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 2011 and prior expired on September 15, 2015. The statute of limitations for the Company's U.S. federal income tax return for 2012 will expire on September 15, 2016, unless otherwise extended. The IRS is currently conducting an examination of the Company's consolidated returns. The Company's state income tax returns remain subject to examination by various state authorities for tax years ranging from 2004 through 2014.
Open tax year	2014