Summary of Significant Accounting Policies - Revenue Recognition/Advertising Costs/Equity Method Investments (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Mar. 31, 2020	Mar. 02, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract liability	$ 62,000,000.0	$ 58,400,000	$ 62,000,000.0	$ 58,400,000	$ 60,400,000
Revenue recognized	11,300,000	13,400,000	31,600,000	32,600,000
Shipping and handling costs	21,700,000	20,500,000	36,500,000	42,200,000
Loss from equity method investments	179,000	4,491,000	5,707,000	4,237,000
Net revenues	$ 707,640,000	1,191,729,000	$ 1,637,880,000	2,396,451,000
Dome Corporation
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Ownership percentage	29.50%		29.50%
Net assets exceeding investment value						$ 3,700,000
Impairment charge			$ 3,700,000
Investment	$ 0	48,600,000	0	48,600,000	5,100,000
Loss from equity method investments	0	4,500,000	1,400,000	4,200,000
Dome Corporation | Licensing Receivable
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net	3,200,000	5,200,000	3,200,000	5,200,000	15,600,000
Dome Corporation | License
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net revenues		$ 5,200,000	6,700,000	11,700,000
UA Sports (Thailand) Co., Ltd
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Ownership percentage							49.50%
Investment	5,000,000.0		5,000,000.0
Loss from equity method investments	$ 200,000		600,000
Customer Refund Liability
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reserves for customer returns allowances markdowns and discounts			199,000,000.0	241,500,000	219,400,000
Prepaid Expenses and Other Current Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reserves for customer returns allowances markdowns and discounts			$ 59,700,000	$ 68,600,000	$ 61,100,000