Summary of Significant Accounting Policies - Revenue Recognition/Advertising Costs/Equity Method Investments (Details) - USD ($)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Mar. 31, 2020	Mar. 02, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract liability	$ 64,900,000		$ 60,600,000	$ 64,900,000	$ 60,600,000	$ 60,400,000
Revenue recognized	9,900,000		9,200,000	21,200,000	22,600,000
Shipping and handling costs	21,200,000		20,800,000	61,200,000	63,000,000.0
Loss from equity method investments	1,199,000		1,177,000	6,906,000	5,414,000
Net revenues	$ 1,433,021,000		1,429,456,000	$ 3,070,901,000	3,825,907,000
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		47,400,000	0	47,400,000	5,100,000
Loss from equity method investments	0		1,200,000	1,400,000	5,400,000
Dome Corporation | Licensing Receivable
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net	8,500,000		9,100,000	8,500,000	9,100,000	15,600,000
Dome Corporation | License
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net revenues	10,900,000	$ 3,000,000.0	$ 9,200,000	17,400,000	20,900,000
UA Sports (Thailand) Co., Ltd
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Ownership percentage								49.50%
Investment	3,700,000			3,700,000
Loss from equity method investments	$ 1,200,000			1,800,000
Customer Refund Liability
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reserves for customer returns allowances markdowns and discounts				197,500,000	209,800,000	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				$ 50,800,000	$ 58,700,000	$ 61,100,000