REVENUE RECOGNITION (Impact of New Revenue Guidance) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Balance Sheet:
Accounts receivable	$ 114.1		$ 114.1
Inventories	962.8		962.8		$ 821.9	$ 826.0
Other current assets	125.5		125.5		135.7	119.3
Accounts payable and accrued liabilities	291.3		291.3			313.2
Deferred income taxes	17.4		17.4		15.6	12.5
Retained earnings	887.1		887.1		$ 759.5	750.3
Statement of Income:
Parts and service	206.1	$ 197.2	609.9	$ 589.5
Finance and insurance, net	73.3	67.7	215.0	202.5
Parts and service	76.3	74.4	225.4	222.3
Income before income taxes	59.1	50.1	170.7	154.7
Income tax expense	14.8	19.4	43.1	58.1
Net income	$ 44.3	$ 30.7	$ 127.6	$ 96.6
Earnings Per Common Share:
Basic (in dollars per share)	$ 2.22	$ 1.49	$ 6.29	$ 4.64
Diluted (in dollars per share)	$ 2.18	$ 1.48	$ 6.22	$ 4.60
Gross profit	$ 278.0	$ 260.3	$ 821.2	$ 787.5
Calculated under Revenue Guidance in Effect before Topic 606
Balance Sheet:
Accounts receivable	123.6		123.6
Inventories	965.5		965.5			826.0
Other current assets	112.2		112.2			119.3
Accounts payable and accrued liabilities	291.7		291.7
Deferred income taxes	14.3		14.3			12.5
Retained earnings	888.7		888.7			$ 750.3
Statement of Income:
Parts and service	205.8		612.0
Finance and insurance, net	73.2		216.0
Parts and service	76.1		226.8
Income before income taxes	58.9		172.4
Income tax expense	14.8		43.5
Net income	$ 44.1		$ 128.9
Earnings Per Common Share:
Basic (in dollars per share)	$ 2.21		$ 6.35
Diluted (in dollars per share)	$ 2.17		$ 6.29
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Balance Sheet:
Accounts receivable	$ (9.5)		$ (9.5)
Inventories	(2.7)		(2.7)
Other current assets	13.3		13.3
Accounts payable and accrued liabilities	(0.4)		(0.4)
Deferred income taxes	3.1		3.1
Retained earnings	(1.6)		(1.6)
Statement of Income:
Parts and service	0.3		(2.1)
Finance and insurance, net	0.1		(1.0)
Parts and service	0.2		(1.4)
Income before income taxes	0.2		(1.7)
Income tax expense	0.0		(0.4)
Net income	$ 0.2		$ (1.3)
Earnings Per Common Share:
Basic (in dollars per share)	$ 0.01		$ (0.06)
Diluted (in dollars per share)	$ 0.01		$ (0.07)
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09 | Open Vehicle Repair Orders
Earnings Per Common Share:
Gross profit	$ 0.1		$ (0.7)