Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)	$ 126.4	$ 38.0	$ 186.4	$ 125.9
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization	19.1	18.2	56.5	53.0
Change in operating lease assets			18.1	21.2
Deferred income taxes			(2.8)	3.6
Asset impairments	0.0	10.3	23.8	10.8
Stock-based compensation			27.0	14.5
Amortization of debt discount and issue costs			2.6	3.1
(Gain) loss on disposition of assets			0.0	(5.9)
(Gain) loss on extinguishment of debt	3.3	0.0	13.7	0.0
Other			1.9	0.7
Changes in assets and liabilities, net of acquisitions and dispositions:
Accounts payable and accrued expenses			(58.8)	99.0
Accounts and notes receivables			25.2	(31.7)
Inventories			499.6	41.7
Contracts-in-transit and vehicle receivables			33.0	8.1
Prepaid expenses and other assets			41.1	(10.6)
Floorplan notes payable — manufacturer affiliates			(137.9)	(1.1)
Deferred revenues			(0.4)	(0.4)
Operating lease liabilities			(16.3)	(21.3)
Net cash provided by (used in) operating activities			712.7	310.8
CASH FLOWS FROM INVESTING ACTIVITIES:
Cash paid in acquisitions, net of cash received			(1.3)	(97.0)
Proceeds from disposition of franchises, property and equipment			1.3	43.1
Purchases of property and equipment			(78.8)	(139.6)
Other			0.0	(0.1)
Net cash provided by (used in) investing activities			(78.8)	(193.5)
CASH FLOWS FROM FINANCING ACTIVITIES:
Borrowings on credit facility — floorplan line and other			7,590.5	5,311.2
Repayments on credit facility — floorplan line and other			(7,960.7)	(5,364.3)
Borrowings on credit facility — acquisition line			284.0	230.5
Repayments on credit facility — acquisition line			(296.5)	(211.4)
Debt issuance costs			(9.0)	(3.2)
Borrowings of senior notes			550.0	0.0
Repayments of senior notes			(857.9)	0.0
Borrowings on other debt			252.9	205.6
Repayments on other debt			(90.8)	(246.5)
Proceeds from employee stock purchase plan			7.0	6.5
Payments of tax withholding for stock-based awards			(5.5)	(3.6)
Repurchases of common stock, amounts based on settlement date			(48.9)	0.0
Dividends paid			(5.5)	(14.8)
Net cash provided by (used in) financing activities			(590.4)	(90.1)
Effect of exchange rate changes on cash			(5.4)	(1.3)
Net increase (decrease) in cash, cash equivalents and restricted cash			38.1	25.9
CASH, CASH EQUIVALENTS AND RESTRICTED CASH, beginning of period			28.1	18.7	$ 18.7
CASH, CASH EQUIVALENTS AND RESTRICTED CASH, end of period	$ 66.2	$ 44.6	$ 66.2	$ 44.6	$ 28.1