Segments - Certain Segment Financial Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 12,672.7	$ 11,821.4	$ 10,086.5
Income before income taxes	375.4	337.5	347.1
Depreciation and amortization	(82.4)	(75.4)	(57.7)
Other interest expense	(60.6)	(56.0)	(34.8)
Other income, net	13.8	8.8	12.2
Floor plan interest expense:
Floor plan interest expense	72.8	62.3	39.3
Operating Segments
Segment Reporting Information [Line Items]
Total revenues	12,642.1	11,813.4	10,088.7
Income before income taxes	334.4	257.7	260.0
Floor plan interest expense:
Floor plan interest expense	127.9	119.6	82.0
Operating Segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	4,382.4	4,215.0	3,845.8
Income before income taxes	123.4	97.6	105.2
Floor plan interest expense:
Floor plan interest expense	53.6	52.4	37.2
Operating Segments | Import
Segment Reporting Information [Line Items]
Total revenues	5,267.8	5,038.1	4,432.8
Income before income taxes	153.9	116.2	117.8
Floor plan interest expense:
Floor plan interest expense	44.1	41.6	29.0
Operating Segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	2,991.9	2,560.3	1,810.1
Income before income taxes	57.1	43.9	37.0
Floor plan interest expense:
Floor plan interest expense	30.2	25.6	15.8
Corporate and other
Segment Reporting Information [Line Items]
Total revenues	30.6	8.0	(2.2)
Income before income taxes	170.2	202.4	167.4
Floor plan interest expense:
Floor plan interest expense	(55.1)	(57.3)	(42.7)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Depreciation and amortization	(82.4)	(75.4)	(57.7)
Other interest expense	(60.6)	(56.0)	(34.8)
Other income, net	$ 13.8	$ 8.8	$ 12.2