Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES:
Net income	$ 396.0	$ 434.6	$ 430.5
Adjustments to reconcile net income to net cash provided by operating activities:
(Income) loss from discontinued operations	(0.1)	0.4	1.2
Depreciation and amortization	166.2	158.6	143.4
Amortization of debt issuance costs and accretion of debt discounts	5.4	5.6	5.4
Stock-based compensation expense	25.5	20.6	25.1
Deferred income tax provision (benefit)	14.5	(19.0)	3.7
Net gain on asset sales and dispositions	(57.6)	(95.4)	(62.6)
Franchise rights impairment	8.1	0.0	0.0
Non-cash impairment charges	3.2	26.4	14.0
Excess tax benefit from stock-based awards	0.0	0.0	(0.6)
Other	0.8	(7.3)	(10.6)
(Increase) decrease, net of effects from business combinations and divestitures:
Receivables	133.7	(61.6)	(99.3)
Inventory	(319.5)	39.3	259.1
Other assets	(107.9)	(37.0)	(33.6)
Increase (decrease), net of effects from business combinations and divestitures:
Vehicle floorplan payable-trade, net	242.4	(64.4)	(196.4)
Accounts payable	1.7	0.5	(5.8)
Other liabilities	(2.0)	139.1	43.8
Net cash provided by continuing operations	510.4	540.4	517.3
Net cash provided by (used in) discontinued operations	0.6	(0.3)	(1.3)
Net cash provided by operating activities	511.0	540.1	516.0
CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of property and equipment	(387.0)	(310.1)	(244.5)
Property operating lease buy-outs	(13.8)	(3.3)	(5.0)
Proceeds from the sale of property and equipment	28.0	21.0	8.7
Proceeds from the disposal of assets held for sale	21.1	38.0	4.8
Insurance recoveries on property and equipment	1.1	1.7	3.1
Cash used in business acquisitions, net of cash acquired	(67.2)	(76.8)	(410.4)
Cash received from business divestitures, net of cash relinquished	173.2	104.6	150.4
Investment in equity security	(50.0)	0.0	0.0
Other	(0.7)	(2.1)	(0.1)
Net cash used in continuing operations	(295.3)	(227.0)	(493.0)
Net cash used in discontinued operations	0.0	0.0	0.0
Net cash used in investing activities	(295.3)	(227.0)	(493.0)
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Repurchases of common stock	(100.0)	(434.9)	(497.0)
Payment of 6.75% Senior Notes due 2018	(400.0)	0.0	0.0
Proceeds from revolving credit facilities	0.0	1,307.0	1,330.0
Payments of revolving credit facilities	0.0	(1,307.0)	(1,330.0)
Net proceeds from (payments of) commercial paper	300.0	(612.0)	342.5
Payment of debt issuance costs	0.0	(13.5)	0.0
Net proceeds from (payments of) vehicle floorplan payable - non-trade	(34.2)	130.2	153.8
Purchase of subsidiary shares	0.0	0.0	(15.2)
Payments of mortgage facilities	0.0	(153.2)	(22.5)
Payments of capital lease and other debt obligations	(15.8)	(11.8)	(4.2)
Proceeds from the exercise of stock options	17.8	39.7	8.4
Payments of tax withholdings for stock-based awards	(2.7)	(1.1)	(2.0)
Excess tax benefit from stock-based awards	0.0	0.0	0.6
Other	(2.5)	0.0	0.0
Net cash used in continuing operations	(237.4)	(307.4)	(35.6)
Net cash used in discontinued operations	0.0	0.0	0.0
Net cash provided by (used in) financing activities	(237.4)	(307.4)	(35.6)
INCREASE (DECREASE) IN CASH, CASH EQUIVALNTS, AND RESTRICTED CASH	(21.7)	5.7	(12.6)
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH at beginning of year	71.1	65.4	78.0
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH at end of year	49.4	71.1	65.4
Senior Notes at Three Point Five Percent Due 2024 [Member]
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Proceeds from Senior Notes	0.0	449.4	0.0
Senior Notes at Three Point Eight Percent Due 2027 [Member]
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Proceeds from Senior Notes	$ 0.0	$ 299.8	$ 0.0