Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES:
Net income	$ 434.6	$ 430.5	$ 442.6
Adjustments to reconcile net income to net cash provided by operating activities:
Loss from discontinued operations	0.4	1.2	1.1
Depreciation and amortization	158.6	143.4	127.4
Amortization of debt issuance costs and accretion of debt discounts	5.6	5.4	4.7
Stock-based compensation expense	20.6	25.1	24.0
Franchise rights impairment	0.0	0.0	15.4
Non-cash impairment charges	26.4	14.0	6.1
Net gain on asset sales and dispositions	(95.4)	(62.6)	(23.8)
Deferred income tax provision	(19.0)	3.7	10.0
Excess tax benefit from stock-based awards	0.0	(0.6)	(17.9)
Other	(7.3)	(10.6)	1.3
(Increase) decrease, net of effects from business combinations and divestitures:
Receivables	(61.6)	(99.3)	(91.8)
Inventory	39.3	259.1	(548.8)
Other assets	(37.0)	(33.6)	(8.8)
Increase (decrease), net of effects from business combinations and divestitures:
Vehicle floorplan payable-trade, net	(64.4)	(196.4)	488.0
Accounts payable	0.5	(5.8)	37.7
Other liabilities	139.1	43.8	41.0
Net cash provided by continuing operations	540.4	517.3	508.2
Net cash used in discontinued operations	(0.3)	(1.3)	(1.0)
Net cash provided by operating activities	540.1	516.0	507.2
CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of property and equipment	(310.1)	(244.5)	(247.6)
Property operating lease buy-outs	(3.3)	(5.0)	(10.2)
Proceeds from the sale of property and equipment	21.0	8.7	21.9
Proceeds from the disposal of assets held for sale	38.0	4.8	11.5
Insurance recoveries on property and equipment	1.7	3.1	1.0
Cash used in business acquisitions, net of cash acquired	(76.8)	(410.4)	(321.5)
Cash received from business divestitures, net of cash relinquished	104.6	150.4	43.9
Net change in restricted cash	(1.3)	3.3	(3.8)
Other	(2.1)	(0.1)	(4.6)
Net cash used in continuing operations	(228.3)	(489.7)	(509.4)
Net cash used in discontinued operations	0.0	0.0	0.0
Net cash used in investing activities	(228.3)	(489.7)	(509.4)
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Repurchases of common stock	(436.0)	(499.0)	(237.3)
Proceeds from revolving credit facilities	1,307.0	1,330.0	1,410.0
Payments of revolving credit facilities	(1,307.0)	(1,330.0)	(2,520.0)
Net proceeds from (payments of) commercial paper	(612.0)	342.5	599.5
Payment of debt issuance costs	(13.5)	0.0	(6.4)
Net proceeds from (payments of) vehicle floorplan payable - non-trade	130.2	153.8	(13.3)
Purchase of subsidiary shares	0.0	(15.2)	0.0
Payments of mortgage facilities	(153.2)	(22.5)	(9.8)
Payments of capital lease and other debt obligations	(11.8)	(4.2)	(18.2)
Proceeds from the exercise of stock options	39.7	8.4	30.0
Excess tax benefit from stock-based awards	0.0	0.6	17.9
Net cash provided by (used in) continuing operations	(307.4)	(35.6)	0.9
Net cash used in discontinued operations	0.0	0.0	0.0
Net cash provided by (used in) financing activities	(307.4)	(35.6)	0.9
INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	4.4	(9.3)	(1.3)
CASH AND CASH EQUIVALENTS at beginning of period	64.8	74.1	75.4
CASH AND CASH EQUIVALENTS at end of period	69.2	64.8	74.1
Senior Notes at Three Point Three Five Percent Due 2021 [Member]
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Proceeds from Senior Notes	0.0	0.0	300.0
Senior Notes at Three Point Five Percent Due 2024 [Member]
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Proceeds from Senior Notes	449.4	0.0	0.0
Senior Notes at Four Point Five Percent Due 2025 [Member]
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Proceeds from Senior Notes	0.0	0.0	448.5
Senior Notes at Three Point Eight Percent Due 2027 [Member]
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Proceeds from Senior Notes	$ 299.8	$ 0.0	$ 0.0