Condensed Consolidated Financial Information - Condensed Consolidated Statements of Operations (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
Condensed Income Statements, Captions [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
COST OF SALES										10,227.8	9,876.3	9,478.2
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
Asset impairments	11.5	10.3	0.5		16.5		4.3		$ 23.2	22.2	43.9	19.5
INCOME (LOSS) FROM OPERATIONS	96.5	83.3	97.1	86.8	74.7	78.2	109.2	79.1		363.7	341.1	341.9
Floorplan interest expense										61.6	59.9	52.4
Other interest expense, net										74.9	75.8	70.5
INCOME (LOSS) BEFORE INCOME TAXES AND EQUITY IN EARNINGS OF SUBSIDIARIES										227.3	205.4	219.0
(Benefit) provision for income taxes										53.3	47.6	5.6
EQUITY IN EARNINGS OF SUBSIDIARIES										0.0	0.0	0.0
NET INCOME (LOSS)	$ 48.1	$ 38.0	$ 49.2	$ 38.6	$ 30.7	$ 34.8	$ 56.5	$ 35.8		174.0	157.8	213.4
COMPREHENSIVE INCOME (LOSS)										(9.2)	(14.4)	23.7
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT										164.8	143.4	237.2
Elimination
Condensed Income Statements, Captions [Line Items]
REVENUES										0.0	0.0	0.0
COST OF SALES										0.0	0.0	0.0
GROSS PROFIT										0.0	0.0	0.0
Selling, general and administrative expenses										0.0	0.0	0.0
Depreciation and amortization										0.0	0.0	0.0
Asset impairments										0.0	0.0	0.0
INCOME (LOSS) FROM OPERATIONS										0.0	0.0	0.0
Floorplan interest expense										0.0	0.0	0.0
Other interest expense, net										0.0	0.0	0.0
INCOME (LOSS) BEFORE INCOME TAXES AND EQUITY IN EARNINGS OF SUBSIDIARIES										0.0	0.0	0.0
(Benefit) provision for income taxes										0.0	0.0	0.0
EQUITY IN EARNINGS OF SUBSIDIARIES										(177.1)	(160.4)	(215.6)
NET INCOME (LOSS)										(177.1)	(160.4)	(215.6)
COMPREHENSIVE INCOME (LOSS)										9.2	0.0	0.0
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT										(167.9)	(160.4)	(215.6)
Group 1 Automotive, Inc. | Reportable Legal Entities
Condensed Income Statements, Captions [Line Items]
REVENUES										0.0	0.0	0.0
COST OF SALES										0.0	0.0	0.0
GROSS PROFIT										0.0	0.0	0.0
Selling, general and administrative expenses										4.1	3.4	3.4
Depreciation and amortization										0.0	0.0	0.0
Asset impairments										0.0	0.0	0.0
INCOME (LOSS) FROM OPERATIONS										(4.1)	(3.4)	(3.4)
Floorplan interest expense										0.0	0.0	0.0
Other interest expense, net										0.0	0.0	0.0
INCOME (LOSS) BEFORE INCOME TAXES AND EQUITY IN EARNINGS OF SUBSIDIARIES										(4.1)	(3.4)	(3.4)
(Benefit) provision for income taxes										(1.0)	(0.8)	(1.3)
EQUITY IN EARNINGS OF SUBSIDIARIES										177.1	160.4	215.6
NET INCOME (LOSS)										174.0	157.8	213.4
COMPREHENSIVE INCOME (LOSS)										(9.2)	0.0	0.0
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT										164.8	157.8	213.4
Guarantor Subsidiaries | Reportable Legal Entities
Condensed Income Statements, Captions [Line Items]
REVENUES										9,184.3	8,723.3	8,680.6
COST OF SALES										7,689.5	7,332.0	7,315.3
GROSS PROFIT										1,494.8	1,391.3	1,365.3
Selling, general and administrative expenses										1,066.6	971.9	974.4
Depreciation and amortization										55.4	52.9	48.3
Asset impairments										14.6	43.4	12.8
INCOME (LOSS) FROM OPERATIONS										358.2	323.1	329.9
Floorplan interest expense										53.8	52.8	47.2
Other interest expense, net										67.5	68.1	66.5
INCOME (LOSS) BEFORE INCOME TAXES AND EQUITY IN EARNINGS OF SUBSIDIARIES										236.9	202.2	216.1
(Benefit) provision for income taxes										56.2	44.5	6.9
EQUITY IN EARNINGS OF SUBSIDIARIES										0.0	0.0	0.0
NET INCOME (LOSS)										180.7	157.7	209.2
COMPREHENSIVE INCOME (LOSS)										(13.0)	9.8	8.7
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT										167.7	167.5	217.9
Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Income Statements, Captions [Line Items]
REVENUES										2,859.5	2,878.1	2,443.2
COST OF SALES										2,538.3	2,544.3	2,163.0
GROSS PROFIT										321.2	333.8	280.2
Selling, general and administrative expenses										287.7	297.7	248.4
Depreciation and amortization										16.2	14.2	9.7
Asset impairments										7.6	0.5	6.7
INCOME (LOSS) FROM OPERATIONS										9.7	21.4	15.4
Floorplan interest expense										7.8	7.1	5.2
Other interest expense, net										7.4	7.7	4.0
INCOME (LOSS) BEFORE INCOME TAXES AND EQUITY IN EARNINGS OF SUBSIDIARIES										(5.5)	6.5	6.2
(Benefit) provision for income taxes										(1.9)	3.9	(0.1)
EQUITY IN EARNINGS OF SUBSIDIARIES										0.0	0.0	0.0
NET INCOME (LOSS)										(3.6)	2.6	6.4
COMPREHENSIVE INCOME (LOSS)										3.8	(24.2)	15.1
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT										$ 0.2	$ (21.5)	$ 21.4