CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Convertible Senior Notes 1.00 Percent Due in 2018	Convertible Senior Notes 1.00 Percent Due in 2018 Common Stock	Convertible Senior Notes 1.00 Percent Due in 2018 Additional Paid-in Capital
Beginning balance (in shares) at Mar. 31, 2018		132,743
Beginning balance, treasury stock (in shares) at Mar. 31, 2018				(18,705)
Beginning balance at Mar. 31, 2018	$ 1,488,970	$ 1,327	$ 1,888,039	$ (458,180)	$ 73,516	$ (15,732)
Increase (Decrease) in Stockholders' Equity
Net income	277,007				277,007
Change in cumulative foreign currency translation adjustment	(40,666)
Change in cumulative foreign currency translation adjustment	(45,320)					(45,320)
Change in unrealized gains on cash flow hedge, net	4,387					4,387
Net unrealized gain on available-for-sale securities, net of taxes	1,058					1,058
Stock-based compensation	161,485		161,485
Repurchased common stock (in shares)				(2,597)
Repurchased common stock	(262,392)			$ (262,392)
Issuance of restricted stock, net of forfeitures and cancellations (in shares)		1,972
Issuance of restricted stock, net of forfeitures and cancellations		$ 19	(19)
Conversion of 1.00% Convertible Notes Due 2018 (in shares)								378
Conversion of 1.00% Convertible Notes Due 2018							$ 8,113	$ 4	$ 8,109
Net share settlement of restricted stock awards (in shares)		(754)
Net share settlement of restricted stock awards	(85,837)	$ (7)	(85,830)
Employee share purchase plan settlement (in shares)		57
Employee share purchase plan settlement	5,070	$ 1	5,069
Ending balance (in shares) at Dec. 31, 2018		134,396
Ending balance, treasury stock (in shares) at Dec. 31, 2018				(21,302)
Ending balance at Dec. 31, 2018	2,027,467	$ 1,344	1,976,853	$ (720,572)	820,796	(50,954)
Beginning balance (in shares) at Sep. 30, 2018		134,106
Beginning balance, treasury stock (in shares) at Sep. 30, 2018				(20,302)
Beginning balance at Sep. 30, 2018	1,944,438	$ 1,341	1,951,128	$ (611,680)	640,848	(37,199)
Increase (Decrease) in Stockholders' Equity
Net income	179,948				179,948
Change in cumulative foreign currency translation adjustment	(16,331)
Change in cumulative foreign currency translation adjustment	(16,331)					(16,331)
Change in unrealized gains on cash flow hedge, net	2,408					2,408
Net unrealized gain on available-for-sale securities, net of taxes	168					168
Stock-based compensation	42,529		42,529
Repurchased common stock (in shares)				(1,000)
Repurchased common stock	(108,892)			$ (108,892)
Issuance of restricted stock, net of forfeitures and cancellations (in shares)		440
Issuance of restricted stock, net of forfeitures and cancellations		$ 4	(4)
Net share settlement of restricted stock awards (in shares)		(207)
Net share settlement of restricted stock awards	(21,871)	$ (2)	(21,869)
Employee share purchase plan settlement (in shares)		57
Employee share purchase plan settlement	5,070	$ 1	5,069
Ending balance (in shares) at Dec. 31, 2018		134,396
Ending balance, treasury stock (in shares) at Dec. 31, 2018				(21,302)
Ending balance at Dec. 31, 2018	$ 2,027,467	$ 1,344	1,976,853	$ (720,572)	820,796	(50,954)
Beginning balance (in shares) at Mar. 31, 2019		134,602
Beginning balance, treasury stock (in shares) at Mar. 31, 2019	(22,421)			(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 unrealized 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		$ 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
Ending balance, treasury stock (in shares) at Dec. 31, 2019	(22,421)			(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
Beginning balance, treasury stock (in shares) at Sep. 30, 2019				(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
Change in cumulative foreign currency translation adjustment	18,172					18,172
Change in unrealized 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		$ 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
Ending balance, treasury stock (in shares) at Dec. 31, 2019	(22,421)			(22,421)
Ending balance at Dec. 31, 2019	$ 2,402,035	$ 1,358	$ 2,100,373	$ (820,572)	$ 1,159,363	$ (38,487)