FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS - Schedule of Impact of Interest Rate Derivative Instruments (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2026 |
Jun. 30, 2025 |
Jun. 30, 2026 |
Jun. 30, 2025 |
|
| Derivative | ||||
| Amount of Unrealized Income (Loss), Net of Tax, Recognized in Other Comprehensive Income (Loss) | $ 3.5 | $ (1.8) | $ 6.4 | $ (6.7) |
| Amount Reclassified from Other Comprehensive Income (Loss) into Statements of Operations | 3.4 | 5.1 | 7.1 | 10.3 |
| Location, Statement of Income, Balance [Axis]: us-gaap:InterestExpenseOther | ||||
| Derivative | ||||
| Amount Reclassified from Other Comprehensive Income (Loss) into Statements of Operations | 1.8 | 2.7 | 3.8 | 5.6 |
| Location, Statement of Income, Balance [Axis]: gpi:FloorPlanInterestExpense | ||||
| Derivative | ||||
| Amount Reclassified from Other Comprehensive Income (Loss) into Statements of Operations | $ 2.7 | $ 4.0 | $ 5.4 | $ 8.0 |