Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES:
Net income	$ 381,600,000	$ 450,000,000.0	$ 396,000,000.0
Adjustments to reconcile net income to net cash provided by operating activities:
(Income) loss from discontinued operations	200,000	800,000	(100,000)
Depreciation and amortization	198,900,000	180,500,000	166,200,000
Amortization of debt issuance costs and accretion of debt discounts	4,700,000	5,200,000	5,400,000
Stock-based compensation expense	30,200,000	31,100,000	25,500,000
Deferred income tax provision (benefit)	(38,900,000)	45,800,000	14,500,000
Net gain on asset sales and dispositions	(7,900,000)	(44,900,000)	(57,600,000)
Goodwill impairment	318,300,000	0	0
Franchise rights impairment	57,500,000	9,600,000	8,100,000
Other non-cash impairment charges	14,700,000	2,200,000	3,200,000
Gain on equity investments	(131,500,000)	(25,700,000)	0
Other	(11,000,000.0)	(6,800,000)	800,000
(Increase) decrease, net of effects from business combinations and divestitures:
Receivables	70,000,000.0	56,200,000	133,700,000
Inventory	703,600,000	296,000,000.0	(319,500,000)
Other assets	85,600,000	49,500,000	(107,900,000)
Increase (decrease), net of effects from business combinations and divestitures:
Vehicle floorplan payable-trade, net	(579,300,000)	(240,600,000)	242,400,000
Accounts payable	47,700,000	(17,600,000)	1,700,000
Other liabilities	63,200,000	(22,100,000)	(2,000,000.0)
Net cash provided by continuing operations	1,207,600,000	769,200,000	510,400,000
Net cash provided by discontinued operations	0	0	600,000
Net cash provided by operating activities	1,207,600,000	769,200,000	511,000,000.0
CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of property and equipment	(156,000,000.0)	(269,300,000)	(400,800,000)
Proceeds from the disposal of assets held for sale	16,400,000	38,000,000.0	21,100,000
Insurance recoveries on property and equipment	1,900,000	3,500,000	1,100,000
Cash used in business acquisitions, net of cash acquired	(400,000)	(4,700,000)	(67,200,000)
Cash received from business divestitures, net of cash relinquished	9,000,000.0	115,600,000	173,200,000
Proceeds from the sale of equity securities	105,400,000	0	0
Investment in equity securities	(50,000,000.0)	0	(50,000,000.0)
Other	0	1,100,000	27,300,000
Net cash used in continuing operations	(73,700,000)	(115,800,000)	(295,300,000)
Net cash used in discontinued operations	0	0	0
Net cash used in investing activities	(73,700,000)	(115,800,000)	(295,300,000)
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Repurchases of common stock	(367,200,000)	(44,700,000)	(100,000,000.0)
Proceeds from revolving credit facilities	1,110,000,000.0	0	0
Payments of revolving credit facilities	(1,110,000,000.0)	0	0
Net proceeds from (payments of) commercial paper	(170,000,000.0)	(460,000,000.0)	300,000,000.0
Payment of debt issuance costs	(11,000,000.0)	0	0
Net payments of vehicle floorplan payable - non-trade	(233,300,000)	(134,300,000)	(34,200,000)
Payment of other debt obligations	(17,200,000)	(31,000,000.0)
Payments of other debt obligations			(15,800,000)
Proceeds from the exercise of stock options	52,700,000	12,700,000	17,800,000
Payments of tax withholdings for stock-based awards	(8,100,000)	(3,000,000.0)	(2,700,000)
Other	0	0	(2,500,000)
Net cash used in continuing operations	(606,700,000)	(660,300,000)	(237,400,000)
Net cash used in discontinued operations	0	0	0
Net cash used in financing activities	(606,700,000)	(660,300,000)	(237,400,000)
INCREASE (DECREASE) IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH	527,200,000	(6,900,000)	(21,700,000)
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH at beginning of year	42,500,000	49,400,000	71,100,000
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH at end of year	569,700,000	42,500,000	49,400,000
Senior Notes at Six Point Seven Five Percent Due 2018
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Payment of Senior Notes	0	0	(400,000,000.0)
Senior Notes at Five Point Five Percent Due 2020 [Member]
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Payment of Senior Notes	(350,000,000.0)	0	0
Senior Notes at Four Point Seven Five Percent Due 2030 [Member] [Member]
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Proceeds from Senior Notes	$ 497,400,000	$ 0	$ 0