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	97,060				97,060
Change in cumulative foreign currency translation adjustment	(24,335)
Change in cumulative foreign currency translation adjustment	(28,989)					(28,989)
Change in unrealized gains on cash flow hedge, net	1,979					1,979
Net unrealized gain on available-for-sale securities, net of taxes	890					890
Stock-based compensation	118,956		118,956
Repurchased common stock (in shares)				(1,597)
Repurchased common stock	(153,500)			$ (153,500)
Issuance of restricted stock, net of forfeitures and cancellations (in shares)		1,532
Issuance of restricted stock, net of forfeitures and cancellations		$ 15	(15)
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)		(547)
Net share settlement of restricted stock awards	(63,966)	$ (5)	(63,961)
Ending balance (in shares) at Sep. 30, 2018		134,106
Ending balance, treasury stock (in shares) at Sep. 30, 2018				(20,302)
Ending balance at Sep. 30, 2018	1,944,439	$ 1,341	1,951,128	$ (611,680)	640,849	(37,199)
Beginning balance (in shares) at Jun. 30, 2018		133,811
Beginning balance, treasury stock (in shares) at Jun. 30, 2018				(20,302)
Beginning balance at Jun. 30, 2018	1,852,205	$ 1,338	1,888,080	$ (611,680)	615,482	(41,015)
Increase (Decrease) in Stockholders' Equity
Net income	25,367
Change in cumulative foreign currency translation adjustment	2,482
Net unrealized gain on available-for-sale securities, net of taxes	481
Ending balance (in shares) at Sep. 30, 2018		134,106
Ending balance, treasury stock (in shares) at Sep. 30, 2018				(20,302)
Ending balance at Sep. 30, 2018	$ 1,944,439	$ 1,341	1,951,128	$ (611,680)	640,849	(37,199)
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	118,095				118,095
Change in cumulative foreign currency translation adjustment	(21,364)					(21,364)
Change in unrealized gains on cash flow hedge, net	3,371					3,371
Net unrealized gain on available-for-sale securities, net of taxes	720					720
Stock-based compensation	96,706		96,706
Issuance of restricted stock, net of forfeitures and cancellations (in shares)		1,503
Issuance of restricted stock, net of forfeitures and cancellations		$ 15	(15)
Net share settlement of restricted stock awards (in shares)		(551)
Net share settlement of restricted stock awards	(61,478)	$ (6)	(61,472)
Employee share purchase plan settlement (in shares)		62
Employee share purchase plan settlement	$ 5,133	$ 1	5,132
Ending balance (in shares) at Sep. 30, 2019		135,616
Ending balance, treasury stock (in shares) at Sep. 30, 2019	(22,421)			(22,421)
Ending balance at Sep. 30, 2019	$ 2,181,763	$ 1,356	2,059,720	$ (820,572)	995,721	(54,462)
Beginning balance (in shares) at Jun. 30, 2019		135,527
Beginning balance, treasury stock (in shares) at Jun. 30, 2019				(22,421)
Beginning balance at Jun. 30, 2019	2,086,327	$ 1,355	2,025,626	$ (820,572)	923,906	(43,988)
Increase (Decrease) in Stockholders' Equity
Net income	71,815				71,815
Change in cumulative foreign currency translation adjustment	(12,567)					(12,567)
Change in unrealized gains on cash flow hedge, net	2,095					2,095
Net unrealized gain on available-for-sale securities, net of taxes	(2)					(2)
Stock-based compensation	43,455		43,455
Issuance of restricted stock, net of forfeitures and cancellations (in shares)		164
Issuance of restricted stock, net of forfeitures and cancellations		$ 2	(2)
Net share settlement of restricted stock awards (in shares)		(75)
Net share settlement of restricted stock awards	$ (9,360)	$ (1)	(9,359)
Ending balance (in shares) at Sep. 30, 2019		135,616
Ending balance, treasury stock (in shares) at Sep. 30, 2019	(22,421)			(22,421)
Ending balance at Sep. 30, 2019	$ 2,181,763	$ 1,356	$ 2,059,720	$ (820,572)	$ 995,721	$ (54,462)