Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 271.5	$ 265.7	$ 245.2
Adjustments to reconcile net income to net cash provided by operating activities:
Asset impairments	2.6	1.3	0.0
Depreciation and amortization	82.4	75.4	57.7
Stock-based compensation	16.2	13.3	11.3
(Gain) loss on disposal of other assets	(0.1)	0.2	(0.4)
Gain from disposal activities	(9.7)	(15.1)	(5.1)
Deferred income taxes	40.1	33.0	(2.8)
(Increase) decrease (net of acquisitions and dispositions):
Trade receivables, net	24.4	4.7	(57.4)
Inventories	(19.7)	(108.9)	(193.1)
Other assets	12.0	(16.0)	(3.1)
Increase (decrease) (net of acquisitions and dispositions):
Floor plan notes payable	100.7	196.9	20.3
Trade payables	(1.8)	15.1	20.0
Accrued liabilities	(38.0)	28.9	37.2
Other long-term liabilities and deferred revenue	18.9	25.2	19.1
Net cash provided by operating activities	499.5	519.7	148.9
Cash flows from investing activities:
Notes receivable issued	12.5	0.0	0.0
Capital expenditures	(124.9)	(158.0)	(105.4)
Proceeds from sales of assets	1.5	3.1	15.3
Cash paid for other investments	(7.2)	(62.7)	(8.6)
Cash paid for acquisitions, net of cash acquired	(366.6)	(373.8)	(460.4)
Proceeds from sales of stores	46.7	34.3	20.9
Net cash used in investing activities	(438.0)	(557.1)	(538.2)
Cash flows from financing activities:
(Repayments) borrowings on floor plan notes payable: non-trade, net	(54.6)	(21.9)	241.5
Borrowings on lines of credit	3,167.0	2,691.4	1,754.5
Repayments on lines of credit	(3,481.6)	(2,499.6)	(1,836.2)
Principal payments on long-term debt, scheduled	(26.0)	(26.5)	(18.2)
Principal payments on long-term debt and finance lease liabilities, other	(11.0)	(26.1)	(50.3)
Proceeds from issuance of long-term debt	420.3	62.1	395.9
Payment of debt issuance costs	(5.8)	(0.4)	(4.7)
Proceeds from issuance of common stock	11.0	10.1	7.5
Repurchase of common stock	(3.2)	(148.9)	(33.8)
Dividends paid	(27.6)	(27.7)	(26.5)
Payments of contingent consideration related to acquisitions	0.0	(0.8)	0.0
Other financing activity	2.4	0.0	(33.4)
Net cash (used in) provided by financing activities	(9.1)	11.7	396.3
(Increase) decrease in cash and cash equivalents	52.4	(25.7)	7.0
Cash and cash equivalents at beginning of year	31.6	57.3	50.3
Cash and cash equivalents at end of year	84.0	31.6	57.3
Supplemental disclosure of cash flow information:
Cash paid during the period for interest	135.8	117.1	68.9
Cash paid during the period for income taxes, net	38.4	32.9	127.3
Floor plan debt paid in connection with store disposals	18.6	33.1	3.7
Supplemental schedule of non-cash activities:
Debt issued in connection with acquisitions	26.4	125.1	1.8
Debt assumed in connection with acquisitions	0.0	10.8	84.3
Issuance of Class A common stock in connection with acquisition	0.0	$ 0.0	$ 2.1
ROU assets obtained in exchange for lease liabilities	$ 260.3