CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net income	$ 770,100,000	$ 282,700,000
Adjustments to reconcile net income to net cash provided by operating activities:
Asset impairments	1,900,000	7,900,000
Depreciation and amortization	91,500,000	67,300,000
Stock-based compensation	25,600,000	17,200,000
Loss on redemption of senior notes	10,300,000	0
Gain on disposal of other assets	(2,500,000)	(500,000)
Loss (gain) on disposal of franchise	5,200,000	(1,400,000)
Unrealized investment loss	22,300,000	0
Deferred income taxes	25,600,000	6,800,000
Amortization of operating lease right-of-use assets	27,000,000.0	20,800,000
(Increase) decrease (net of acquisitions and dispositions):
Accounts receivable, net	(85,000,000.0)	(11,300,000)
Inventories	1,003,200,000	457,000,000.0
Other assets	(351,100,000)	(36,400,000)
Increase (decrease) (net of acquisitions and dispositions):
Floor plan notes payable	91,300,000	(128,000,000.0)
Trade payables	97,200,000	25,000,000.0
Accrued liabilities	236,900,000	66,800,000
Other long-term liabilities and deferred revenue	19,700,000	26,700,000
Net cash provided by operating activities	1,989,200,000	800,600,000
Cash flows from investing activities:
Capital expenditures	(194,100,000)	(125,600,000)
Proceeds from sales of assets	4,600,000	5,100,000
Cash paid for other investments	(9,800,000)	(9,900,000)
Cash paid for acquisitions, net of cash acquired	(2,409,500,000)	(609,500,000)
Proceeds from sales of stores	45,700,000	11,600,000
Net cash used in investing activities	(2,563,100,000)	(728,300,000)
Cash flows from financing activities:
Repayments on floor plan notes payable, net: non-trade	(840,900,000)	(317,800,000)
Borrowings on lines of credit	1,642,500,000	1,740,500,000
Repayments on lines of credit	(1,631,600,000)	(1,485,000,000)
Principal payments on long-term debt and finance lease liabilities, scheduled	(24,700,000)	(20,700,000)
Principal payments on long-term debt and finance lease liabilities, other	(483,600,000)	(6,300,000)
Proceeds from issuance of long-term debt	817,500,000	56,500,000
Payment of debt issuance costs	(10,900,000)	(4,100,000)
Proceeds from issuance of common stock	1,127,900,000	9,200,000
Repurchase of common stock	(15,900,000)	(50,600,000)
Dividends paid	(28,200,000)	(20,900,000)
Payment of contingent consideration related to acquisitions	(1,400,000)	0
Net cash provided by (used in) financing activities	550,700,000	(99,200,000)
Effect of exchange rate changes on cash and cash equivalents	900,000	0
Decrease in cash and cash equivalents	(22,300,000)	(26,900,000)
Cash and cash equivalents at beginning of period	160,100,000	84,000,000.0
Cash and cash equivalents at end of period	137,800,000	57,100,000
Supplemental disclosure of cash flow information:
Cash paid during the period for interest	89,500,000	82,700,000
Cash paid during the period for income taxes, net	237,300,000	74,100,000
Floor plan debt paid in connection with store disposals	8,700,000	22,000,000.0
Supplemental schedule of non-cash activities:
Debt issued in connection with acquisitions	355,400,000	0
Contingent consideration in connection with acquisitions	900,000	9,700,000
Debt assumed in connection with acquisitions	4,000,000.0	0
Right-of-use assets obtained in exchange for lease liabilities	$ 131,200,000	$ 44,100,000