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Reclassification from AOCI, Current Period [Abstract]

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount before tax and reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Amount of tax expense (benefit) allocated to other comprehensive income (loss) before reclassification adjustment from accumulated other comprehensive income (loss).

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Increase (Decrease) in AOCI
Beginning balance	$ 2,839.6	$ 2,992.5	$ 2,789.1	$ 2,974.3
Other comprehensive income (loss) before reclassifications:
Pre-tax			(1.1)	60.9
Tax effect			(2.0)	2.1
Amount reclassified from accumulated other comprehensive income (loss):
Provision for income taxes			2.4	3.2
OTHER COMPREHENSIVE INCOME (LOSS)	1.6	38.9	(10.7)	52.7
Ending balance	2,952.2	3,136.0	2,952.2	3,136.0
Interest Rate Swaps
Amount reclassified from accumulated other comprehensive income (loss):
Pre-tax			(0.8)
Location, Statement of Other Comprehensive Income, Balance [Axis]: us-gaap:InterestExpenseOther
Amount reclassified from accumulated other comprehensive income (loss):
Pre-tax			(3.8)	(5.6)
Location, Statement of Other Comprehensive Income, Balance [Axis]: gpi:FloorPlanInterestExpense
Amount reclassified from accumulated other comprehensive income (loss):
Pre-tax			(5.4)	(8.0)
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in AOCI
Beginning balance	19.3	15.4	31.6	1.6
Amount reclassified from accumulated other comprehensive income (loss):
OTHER COMPREHENSIVE INCOME (LOSS)	1.6	38.9	(10.7)	52.7
Ending balance	20.9	54.4	20.9	54.4
Accumulated Income (Loss) On Foreign Currency Translation
Increase (Decrease) in AOCI
Beginning balance			(1.7)	(56.5)
Other comprehensive income (loss) before reclassifications:
Pre-tax			(9.4)	69.8
Tax effect			0.0	0.0
Amount reclassified from accumulated other comprehensive income (loss):
Provision for income taxes			0.0	0.0
OTHER COMPREHENSIVE INCOME (LOSS)			(9.4)	69.8
Ending balance	(11.1)	13.3	(11.1)	13.3
Accumulated Income (Loss) On Foreign Currency Translation \| Interest Rate Swaps
Amount reclassified from accumulated other comprehensive income (loss):
Pre-tax			0.0
Accumulated Income (Loss) On Foreign Currency Translation \| Location, Statement of Other Comprehensive Income, Balance [Axis]: us-gaap:InterestExpenseOther
Amount reclassified from accumulated other comprehensive income (loss):
Pre-tax			0.0	0.0
Accumulated Income (Loss) On Foreign Currency Translation \| Location, Statement of Other Comprehensive Income, Balance [Axis]: gpi:FloorPlanInterestExpense
Amount reclassified from accumulated other comprehensive income (loss):
Pre-tax			0.0	0.0
Accumulated Income (Loss) On Interest Rate Swaps
Increase (Decrease) in AOCI
Beginning balance			33.3	58.2
Other comprehensive income (loss) before reclassifications:
Pre-tax			8.3	(8.8)
Tax effect			(2.0)	2.1
Amount reclassified from accumulated other comprehensive income (loss):
Provision for income taxes			2.4	3.2
OTHER COMPREHENSIVE INCOME (LOSS)			(1.3)	(17.1)
Ending balance	$ 32.0	$ 41.1	32.0	41.1
Accumulated Income (Loss) On Interest Rate Swaps \| Interest Rate Swaps
Amount reclassified from accumulated other comprehensive income (loss):
Pre-tax			(0.8)
Accumulated Income (Loss) On Interest Rate Swaps \| Location, Statement of Other Comprehensive Income, Balance [Axis]: us-gaap:InterestExpenseOther
Amount reclassified from accumulated other comprehensive income (loss):
Pre-tax			(3.8)	(5.6)
Accumulated Income (Loss) On Interest Rate Swaps \| Location, Statement of Other Comprehensive Income, Balance [Axis]: gpi:FloorPlanInterestExpense
Amount reclassified from accumulated other comprehensive income (loss):
Pre-tax			$ (5.4)	$ (8.0)