CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 50,091	$ 49,487
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	7,891	7,880
Stock-based compensation	10,953	9,482
Provision for excess and obsolete inventories	3,281	899
Deferred income taxes	5,293	(902)
Excess tax benefit from stock-based compensation	(4,086)	(679)
Other operating activities	1,200	1,265
Changes in assets and liabilities:
Accounts receivable, net	3,082	5,008
Inventory, net	(4,552)	(8,230)
Current and other assets	(659)	(1,218)
Accounts payable	(7,567)	(3,854)
Accrued liabilities	(3,885)	(559)
Income taxes	(3,436)	1,445
Cash provided by operating activities	57,606	60,024
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sales of short-term investments	50,118	25,057
Proceeds from maturities of short-term investments	54,970	60,890
Purchase of short-term investments	(41,634)	(65,411)
Proceeds from sales of long-term investments	15,012	2,000
Purchase of long-term investments	(74,720)	(33,951)
Acquisitions, net of cash acquired	0	(1,723)
Capital expenditures	(27,213)	(21,526)
Cash used for investing activities	(23,467)	(34,664)
CASH FLOWS FROM FINANCING ACTIVITIES
Repurchase of common stock	(27,313)	(19,930)
Proceeds from issuances under stock-based compensation plans	18,637	11,888
Employees' tax withheld and paid for restricted stock and restricted stock units	(4,369)	(1,729)
Proceeds from revolving line of credit	0	18,000
Repayments of revolving line of credit	0	(26,000)
Payment of cash dividends	(8,765)	(8,490)
Excess tax benefit from stock-based compensation	4,086	679
Cash used for financing activities	(17,724)	(25,582)
Effect of exchange rate changes on cash and cash equivalents	789	(1,184)
Net increase (decrease) in cash and cash equivalents	17,204	(1,406)
Cash and cash equivalents at beginning of period	228,776	209,335
Cash and cash equivalents at end of period	245,980	207,929
Transfers of long-term investments to short-term investments	$ 60,424	$ 48,574