BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ / shares in Units, $ in Millions					3 Months Ended		6 Months Ended
	Sep. 14, 2020 $ / shares shares	Jun. 30, 2020 shares	Jun. 09, 2020 USD ($) shares	Apr. 25, 2019	Sep. 12, 2020 USD ($) store_format shares	Sep. 07, 2019 USD ($)	Sep. 12, 2020 USD ($) division store_format segment shares	Sep. 07, 2019 USD ($)	Feb. 29, 2020 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restricted cash					$ 42.3		$ 42.3		$ 8.2
LIFO expense					10.1	$ 5.8	23.2	$ 16.3
Equity-based compensation expense					1.3	0.9	2.5	2.1
Repurchase of common stock			$ 1,680.0				1,680.0	0.0
Income tax expense					$ 111.2	$ 81.9	$ 313.1	$ 97.6
Effective tax rate					28.10%	21.70%	26.40%	22.10%
Number of divisions | division							12
Number of reportable segments | segment							1
Number of store format | store_format					1		1
Receivables, net					$ 547.1		$ 547.1		525.3
Contract liability related to gift cards					59.9		59.9		52.2
Deferred employer paid portion of social security taxes							270.0
Pharmacy
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Receivables, net					259.2		259.2		$ 218.5
Common Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of shares repurchased (in shares) | shares			101,611,736
Class A Common Stock | Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Repurchase of common stock (in shares) | shares	6,837,970
Repurchase of common stock per share (in usd per share) | $ / shares	$ 12.00
Phantom Units
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Awards converted (in shares) | shares			11,300,000
Equity-based compensation expense					8.0	$ 5.6	25.8	$ 15.5
Income tax benefit					2.0	$ 1.5	6.6	$ 4.1
Compensation cost not yet recognized					$ 98.8		$ 98.8
Number of unvested phantom units (in shares) | shares					9,800,000		9,800,000
Period for recognition of unrecognized compensation cost							2 years 1 month 28 days
Phantom Units | Investor Incentive Units
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Units received upon vesting (in shares) | shares							1
Restricted Stock Units (RSU) Deemed not Granted
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Awards converted (in shares) | shares			1,900,000
Direct Equity Interest | President and Chief Executive Officer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Award vesting period				5 years
Direct Equity Interest | President and Chief Executive Officer | Award Based on Service Period
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Vesting percentage				50.00%
Direct Equity Interest | President and Chief Executive Officer | Award Based on Service Period and Achievement of Certain Performance Based Thresholds
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Vesting percentage				50.00%
RSUs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Compensation cost not yet recognized					$ 8.7		$ 8.7
Number of unvested phantom units (in shares) | shares					1,100,000		1,100,000
Period for recognition of unrecognized compensation cost							2 years 9 months 7 days
RSUs | IPO
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Performance-based restricted common stock (in shares) | shares		1,700,000
Restricted Stock Units (RSU), Performance Based, Deemed Not Granted | IPO
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Performance-based restricted common stock (in shares) | shares		600,000