Segment Information - Reportable Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 3,111.4	$ 3,118.3	$ 3,005.7	$ 2,808.4	$ 2,908.8	$ 2,889.1	$ 2,943.5	$ 2,860.0		$ 12,043.8	$ 11,601.4	$ 11,123.7
Gross profit	464.6	465.6	454.3	431.5	432.1	435.1	438.2	$ 419.8		1,816.0	1,725.1	1,645.5
Selling, general and administrative expenses										1,358.4	1,273.1	1,226.2
Depreciation and amortization										71.6	67.1	57.9
Floorplan interest expense										61.6	59.9	52.4
Other interest expense, net										74.9	75.8	70.5
Income (loss) before income taxes										227.3	205.4	219.0
Real estate related capital expenditures										92.5	31.4	110.4
Non-real estate related capital expenditures										99.3	109.6	105.4
Total capital expenditures										191.8	141.0	215.8
Loss due to catastrophic events		11.9	4.0	$ 2.0	0.6		5.8			16.0	6.1	5.0
Asset impairments	$ 11.5	$ 10.3	$ 0.5		16.5		4.3		$ 23.2	22.2	43.9	19.5
Decrease (increase) in accrual for capital expenditures										4.1	0.5	7.1
Non-cash capital accruals										5.2	9.2
Gain on disposition on real estate and dealership transactions					$ (0.3)	5.4	20.1			5.0	24.4
Gain on legal settlements						$ 3.1	$ 0.5
U.S.
Segment Reporting Information [Line Items]
Revenues										9,184.2	8,723.3	8,680.6
Gross profit										1,494.8	1,391.3	1,365.3
Selling, general and administrative expenses										1,075.6	982.1	984.0
Depreciation and amortization										55.4	52.9	48.3
Floorplan interest expense										53.7	52.8	47.2
Other interest expense, net										67.5	68.1	66.5
Income (loss) before income taxes										227.9	192.1	206.6
Real estate related capital expenditures										63.8	20.5	101.8
Non-real estate related capital expenditures										70.7	80.2	84.3
Total capital expenditures										134.5	100.7	186.1
Loss due to catastrophic events										17.8	6.4	15.3
Asset impairments										14.7	43.4	12.8
Gain on disposition on real estate and dealership transactions											25.2
U.K.
Segment Reporting Information [Line Items]
Revenues										2,413.7	2,437.4	1,986.0
Gross profit										267.7	279.9	225.3
Selling, general and administrative expenses										236.9	240.4	191.6
Depreciation and amortization										14.6	12.6	8.2
Floorplan interest expense										7.2	6.3	4.7
Other interest expense, net										7.3	6.8	3.7
Income (loss) before income taxes										(5.3)	13.3	17.1
Real estate related capital expenditures										25.7	5.0	1.7
Non-real estate related capital expenditures										25.9	27.5	20.9
Total capital expenditures										51.6	32.5	22.6
Asset impairments										7.0	0.5
Brazil
Segment Reporting Information [Line Items]
Revenues										445.9	440.7	457.2
Gross profit										53.5	53.9	54.9
Selling, general and administrative expenses										46.0	50.6	50.7
Depreciation and amortization										1.6	1.6	1.5
Floorplan interest expense										0.7	0.8	0.4
Other interest expense, net										0.1	0.9	0.3
Income (loss) before income taxes										4.6	0.0	(4.7)
Real estate related capital expenditures										3.1	5.8	6.9
Non-real estate related capital expenditures										2.6	2.0	0.2
Total capital expenditures										5.7	7.8	7.1
Asset impairments										$ 0.5		$ 6.7
Gain on legal settlements											$ 3.7