CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning balance (in shares) at Mar. 31, 2019		134,602		(22,421)
Beginning balance at Mar. 31, 2019	$ 2,040,580	$ 1,346	$ 2,019,369	$ (820,572)	$ 877,626	$ (37,189)
Increase (Decrease) in Stockholders' Equity
Net income	281,737				281,737
Change in cumulative foreign currency translation adjustment	(3,192)					(3,192)
Change in gains on cash flow hedge, net	1,441					1,441
Net unrealized gain on available-for-sale securities, net of taxes	453					453
Stock-based compensation	141,761		141,761
Issuance of restricted stock, net of forfeitures and cancellations (in shares)		1,674
Issuance of restricted stock, net of forfeitures and cancellations	0	$ 17	(17)
Net share settlement of restricted stock awards (in shares)		(631)
Net share settlement of restricted stock awards	(71,260)	$ (7)	(71,253)
Employee share purchase plan settlement (in shares)		126
Employee share purchase plan settlement	10,515	$ 2	10,513
Ending balance (in shares) at Dec. 31, 2019		135,771		(22,421)
Ending balance at Dec. 31, 2019	2,402,035	$ 1,358	2,100,373	$ (820,572)	1,159,363	(38,487)
Beginning balance (in shares) at Sep. 30, 2019		135,616		(22,421)
Beginning balance at Sep. 30, 2019	2,181,763	$ 1,356	2,059,720	$ (820,572)	995,721	(54,462)
Increase (Decrease) in Stockholders' Equity
Net income	163,642				163,642
Change in cumulative foreign currency translation adjustment	18,172					18,172
Change in gains on cash flow hedge, net	(1,930)					(1,930)
Net unrealized gain on available-for-sale securities, net of taxes	(267)					(267)
Stock-based compensation	45,055		45,055
Issuance of restricted stock, net of forfeitures and cancellations (in shares)		171
Issuance of restricted stock, net of forfeitures and cancellations	0	$ 2	(2)
Net share settlement of restricted stock awards (in shares)		(80)
Net share settlement of restricted stock awards	(9,782)	$ (1)	(9,781)
Employee share purchase plan settlement (in shares)		64
Employee share purchase plan settlement	5,382	$ 1	5,381
Ending balance (in shares) at Dec. 31, 2019		135,771		(22,421)
Ending balance at Dec. 31, 2019	2,402,035	$ 1,358	2,100,373	$ (820,572)	1,159,363	(38,487)
Beginning balance (in shares) at Mar. 31, 2020		135,927		(22,421)
Beginning balance at Mar. 31, 2020	2,539,244	$ 1,359	2,134,748	$ (820,572)	1,282,085	(58,376)
Increase (Decrease) in Stockholders' Equity
Net income	370,074				370,074
Change in cumulative foreign currency translation adjustment	53,697					53,697
Change in gains on cash flow hedge, net	(4,905)					(4,905)
Net unrealized gain on available-for-sale securities, net of taxes	4,000					4,000
Stock-based compensation	141,904		141,904
Issuance of restricted stock, net of forfeitures and cancellations (in shares)		1,235
Issuance of restricted stock, net of forfeitures and cancellations	1	$ 13	(12)
Net share settlement of restricted stock awards (in shares)		(401)
Net share settlement of restricted stock awards	(60,586)	$ (5)	(60,581)
Employee share purchase plan settlement (in shares)		139
Employee share purchase plan settlement	14,215	$ 2	14,213
Issuance of shares related to Playdots, Inc. acquisition (in shares)		604
Issuance of shares related to Playdots, Inc. acquisition	97,647	$ 6	97,641
Ending balance (in shares) at Dec. 31, 2020		137,504		(22,421)
Ending balance at Dec. 31, 2020	3,155,291	$ 1,375	2,327,913	$ (820,572)	1,652,159	(5,584)
Beginning balance (in shares) at Sep. 30, 2020		137,349		(22,421)
Beginning balance at Sep. 30, 2020	2,901,282	$ 1,373	2,285,394	$ (820,572)	1,469,911	(34,824)
Increase (Decrease) in Stockholders' Equity
Net income	182,248				182,248
Change in cumulative foreign currency translation adjustment	30,135					30,135
Change in gains on cash flow hedge, net	(600)					(600)
Net unrealized gain on available-for-sale securities, net of taxes	(295)					(295)
Stock-based compensation	47,192		47,192
Issuance of restricted stock, net of forfeitures and cancellations (in shares)		141
Issuance of restricted stock, net of forfeitures and cancellations	1	$ 2	(1)
Net share settlement of restricted stock awards (in shares)		(60)
Net share settlement of restricted stock awards	(12,384)	$ (1)	(12,383)
Employee share purchase plan settlement (in shares)		74
Employee share purchase plan settlement	7,712	$ 1	7,711
Ending balance (in shares) at Dec. 31, 2020		137,504		(22,421)
Ending balance at Dec. 31, 2020	$ 3,155,291	$ 1,375	$ 2,327,913	$ (820,572)	$ 1,652,159	$ (5,584)