CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Registered Shares	Additional Paid-in Capital	Treasury Shares	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning of the period (in shares) at Mar. 31, 2016		173,106		10,697
Beginning of the period at Mar. 31, 2016	$ 759,948	$ 30,148	$ 6,616	$ (128,407)	$ 963,576	$ (111,985)
Increase (Decrease) in Shareholders' Equity
Total comprehensive income	$ 166,004				166,451	(447)
Purchase of registered shares (in shares)	3,300			3,321
Purchases of registered shares	$ (63,764)			$ (63,764)
Tax effects from share-based awards	(1,463)		(1,463)
Sales of shares upon exercise of stock options and purchase rights	20,355		6,435	$ 13,920
Sales of shares upon exercise of options and purchase rights (in shares)				(1,524)
Issuance of shares upon vesting of restricted stock units	(13,054)		(21,714)	$ 10,909	(2,249)
Issuance of shares upon vesting of restricted stock units (in shares)				(1,196)
Share-based compensation	26,462		26,462
Cash dividends	(93,093)				(93,093)
End of the period (in shares) at Dec. 31, 2016		173,106		11,298
End of the period at Dec. 31, 2016	801,395	$ 30,148	16,336	$ (167,342)	1,034,685	(112,432)
Increase (Decrease) in Shareholders' Equity
Cumulative effect of adoption of new accounting standard (Note 1)	57,205		3,293		53,912
Beginning of the period (in shares) at Mar. 31, 2017		173,106		10,727
Beginning of the period at Mar. 31, 2017	856,111	$ 30,148	26,596	$ (174,037)	1,074,110	(100,706)
Increase (Decrease) in Shareholders' Equity
Total comprehensive income	$ 179,677				174,138	5,539
Purchase of registered shares (in shares)	600			581
Purchases of registered shares	$ (20,408)			$ (20,408)
Sales of shares upon exercise of stock options and purchase rights	30,947		15,958	$ 14,989
Sales of shares upon exercise of options and purchase rights (in shares)				(1,126)
Issuance of shares upon vesting of restricted stock units	(25,505)		(40,402)	$ 14,897	0
Issuance of shares upon vesting of restricted stock units (in shares)				(1,283)
Share-based compensation	33,457		33,457
Cash dividends	(104,248)				(104,248)
End of the period (in shares) at Dec. 31, 2017		173,106		8,899
End of the period at Dec. 31, 2017	$ 1,007,236	$ 30,148	$ 38,902	$ (164,559)	$ 1,197,912	$ (95,167)