Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net income		$ 282.7	$ 203.5
Adjustments to reconcile net income to net cash provided by operating activities:
Asset impairments		7.9	0.5
Depreciation and amortization		67.3	60.9
Stock-based compensation		17.2	11.8
Gain on disposal of other assets		(0.5)	0.0
Gain on disposal of franchise		(1.4)	(9.1)
Deferred income taxes		6.8	39.5
(Increase) decrease (net of acquisitions and dispositions):
Accounts receivable, net		(11.3)	69.7
Inventories		457.0	(4.1)
Other assets		7.8	22.3
Increase (decrease) (net of acquisitions and dispositions):
Floor plan notes payable		(128.0)	83.1
Trade payables		25.0	0.1
Accrued liabilities		23.6	(36.5)
Other long-term liabilities and deferred revenue		46.5	11.8
Net cash provided by operating activities		800.6	453.5
Cash flows from investing activities:
Capital expenditures		(125.6)	(91.9)
Proceeds from sales of assets		5.1	0.8
Cash paid for other investments		(9.9)	(6.7)
Cash paid for acquisitions, net of cash acquired		(609.5)	(142.8)
Proceeds from sales of stores		11.6	40.9
Net cash used in investing activities		(728.3)	(199.7)
Cash flows from financing activities:
Repayments on floor plan notes payable, net: non-trade		(317.8)	(114.0)
Borrowings on lines of credit		1,740.5	2,147.0
Repayments on lines of credit		(1,485.0)	(2,209.6)
Principal payments on long-term debt and capital leases, scheduled		(20.7)	(18.0)
Principal payments on long-term debt and capital leases, other		(6.3)	(11.0)
Proceeds from issuance of long-term debt		56.5	0.0
Payments of debt issuance costs		(4.1)	(0.4)
Proceeds from issuance of common stock		9.2	8.0
Repurchase of common stock		(50.6)	(3.1)
Dividends paid		(20.9)	(20.7)
Other financing activity		0.0	(36.5)
Net cash used in financing activities		(99.2)	(258.3)
Decrease in cash and cash equivalents		(26.9)	(4.5)
Cash and cash equivalents at beginning of period		84.0	31.6
Cash and cash equivalents at end of period		57.1	27.1
Supplemental disclosure of cash flow information:
Cash paid during the period for interest		82.7	107.6
Cash paid during the period for income taxes, net		74.1	13.8
Floor plan debt paid in connection with store disposals		22.0	12.3
Supplemental schedule of non-cash activities:
Debt issued in connection with acquisitions		0.0	26.4
Contingent consideration in connection with acquisitions		9.7	0.0
ROU assets obtained in exchange for lease liabilities	[1]	$ 44.1	260.3
Acquisition of finance leases in connection with acquisitions			$ 0.0

[1] The amount for the nine months ended September 30, 2019 includes the transition adjustment for the adoption of Topic 842.