Summary of Significant Accounting Policies - Revenue Recognition/Advertising Costs/Equity Method Investments (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Mar. 02, 2020	Apr. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract liability	$ 58,500	$ 55,600	$ 60,400
Revenue recognized	20,300	19,200
Shipping and handling costs	14,900	21,700
Income (loss) from equity method investments	(1,400)	300
Net revenues	930,240	1,204,722
Licensing Receivable
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net	7,000	6,500	15,600
Dome Corporation
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Ownership percentage					29.50%
Net assets exceeding investment value	(3,700)
Impairment charge	3,700
Income (loss) from equity method investments	(5,528)	254
Investment	0		53,100
Dome Corporation | License
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net revenues	6,700	6,500
UA Sports (Thailand) Co., Ltd
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Ownership percentage				49.50%
Income (loss) from equity method investments	(400)
Investment	4,700
Customer Refund Liability
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reserves for customer returns allowances markdowns and discounts	208,200	270,600	$ 219,400
Prepaid Expenses and Other Current Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reserves for customer returns allowances markdowns and discounts	$ 63,300	$ 91,800