CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	$ 602.5	$ 997.3	$ 532.4
Adjustments to reconcile net income to net cash provided by operating activities—
Depreciation and amortization	67.7	69.0	41.9
Share-based compensation	23.5	20.6	16.2
Deferred income taxes	39.7	148.5	31.2
Asset impairments	117.2	0.0	0.0
Unrealized (gain) loss on investments	(2.1)	14.1	(1.0)
Loaner vehicle amortization	34.8	14.7	20.9
Gain on divestitures, net	(13.5)	(207.1)	(8.0)
Change in right-of-use asset	26.8	25.3	22.3
Other adjustments, net	(1.9)	4.6	(0.8)
Changes in operating assets and liabilities, net of acquisitions and divestitures—
Contracts-in-transit	(58.9)	(17.0)	48.5
Accounts receivable	(54.6)	47.6	35.3
Inventories	(144.5)	6.9	670.5
Other current assets	(564.6)	(353.7)	(227.1)
Floor plan notes payable—trade, net	144.1	13.8	(27.6)
Deferred revenue	22.8	42.9	3.6
Accounts payable and accrued liabilities	119.5	(69.8)	39.2
Operating lease liabilities	(26.7)	(25.4)	(20.6)
Other long-term assets and liabilities, net	(18.8)	(36.3)	(13.2)
Net cash provided by operating activities	313.0	696.0	1,163.7
CASH FLOW FROM INVESTING ACTIVITIES:
Capital expenditures—excluding real estate	(142.3)	(94.6)	(74.2)
Capital expenditures—real estate	0.0	(13.3)	(7.8)
Purchases of previously leased real estate	0.0	0.0	(217.1)
Acquisitions, net of cash acquired	(1,500.0)	(5.0)	(3,660.4)
Proceeds from dealership divestitures	30.7	701.2	21.3
Purchases of debt securities—available-for-sale	(195.2)	(202.2)	(1.1)
Purchases of equity securities	0.0	(41.4)	(0.4)
Proceeds from the sale of debt securities—available-for-sale	60.3	69.7	0.8
Proceeds from the sale of equity securities	51.8	50.3	0.4
Proceeds from the sale of assets	16.3	0.0	21.5
Net cash (used in) provided by investing activities	(1,678.4)	464.7	(3,917.0)
CASH FLOW FROM FINANCING ACTIVITIES:
Floor plan borrowings—non-trade	8,385.8	7,406.5	5,042.8
Floor plan borrowings—acquisitions	256.1	0.0	214.5
Floor plan repayments—non-trade	(7,059.8)	(7,891.6)	(5,357.5)
Floor plan repayments—divestitures	0.0	(48.4)	(0.8)
Proceeds from borrowings	0.0	0.0	2,274.0
Repayments of borrowings	(126.0)	(106.2)	(41.5)
Proceeds from revolving credit facility	329.0	330.0	439.0
Repayments of revolving credit facility	(329.0)	(499.0)	(270.0)
Proceeds from issuance of common stock	0.0	1.4	666.9
Payment of debt issuance costs	(1.2)	(0.4)	(26.2)
Purchase of treasury stock	(267.7)	287.4	0.0
Repurchases of common stock, including amounts associated with net share settlements of employee share-based awards	(11.4)	(9.2)	(10.4)
Net cash provided by (used in) financing activities	1,175.8	(1,104.3)	2,930.8
Net (decrease) increase in cash and cash equivalents	(189.6)	56.4	177.5
CASH AND CASH EQUIVALENTS, beginning of period	235.3	178.9	1.4
CASH AND CASH EQUIVALENTS, end of period	$ 45.7	$ 235.3	$ 178.9