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Cover - USD ($)
12 Months Ended
Dec. 31, 2020
Jan. 31, 2021
Jun. 30, 2020
Entity Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2020    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 001-33202    
Entity Registrant Name UNDER ARMOUR, INC.    
Entity Incorporation, State or Country Code MD    
Entity Tax Identification Number 52-1990078    
Entity Address, Address Line One 1020 Hull Street    
Entity Address, City or Town Baltimore    
Entity Address, State or Province MD    
Entity Address, Postal Zip Code 21230    
City Area Code 410    
Local Phone Number 454-6428    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Documents Incorporated by Reference Portions of Under Armour, Inc.’s Proxy Statement for the Annual Meeting of Stockholders to be held on May 13, 2021 are incorporated by reference in Part III of this Form 10-K.    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Central Index Key 0001336917    
Class A Common Stock      
Entity Information [Line Items]      
Title of 12(b) Security Class A Common Stock    
Trading Symbol UAA    
Security Exchange Name NYSE    
Entity Public Float     $ 1,827,569,777
Entity Common Stock, Shares Outstanding   188,619,343  
Class C Common Stock      
Entity Information [Line Items]      
Title of 12(b) Security Class C Common Stock    
Trading Symbol UA    
Security Exchange Name NYSE    
Entity Public Float     $ 2,007,651,870
Entity Common Stock, Shares Outstanding   231,983,924  
Class B Convertible Common Stock      
Entity Information [Line Items]      
Entity Common Stock, Shares Outstanding   34,450,000