CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 56,093	$ 50,091
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	9,088	7,891
Stock-based compensation	13,692	10,953
Provision for excess and obsolete inventories	565	3,281
Deferred income taxes	1,769	5,293
Excess tax benefit from stock-based compensation	(1,684)	(4,086)
Other operating activities	(1,104)	1,200
Changes in assets and liabilities, net of effect of acquisition:
Accounts receivable, net	(1,632)	3,082
Inventory, net	(5,119)	(4,552)
Current and other assets	(2,931)	(659)
Accounts payable	8,158	(7,567)
Accrued liabilities	(7,771)	(3,885)
Income taxes	2,907	(3,436)
Cash provided by operating activities	72,031	57,606
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sales of investments	20,951	65,130
Proceeds from maturities of investments	81,275	54,970
Purchase of investments	(99,225)	(116,354)
Acquisition, net of cash acquired	(150)	0
Capital expenditures	(13,419)	(27,213)
Cash used for investing activities	(10,568)	(23,467)
CASH FLOWS FROM FINANCING ACTIVITIES
Repurchase of common stock	(18,917)	(27,313)
Proceeds from issuances under stock-based compensation plans	11,424	18,637
Employees' tax withheld and paid for restricted stock and restricted stock units	(6,235)	(4,369)
Payment of cash dividends	(12,836)	(8,765)
Excess tax benefit from stock-based compensation	1,684	4,086
Cash used for financing activities	(24,880)	(17,724)
Effect of exchange rate changes on cash and cash equivalents	(1,058)	789
Net increase in cash and cash equivalents	35,525	17,204
Cash and cash equivalents at beginning of period	232,704	228,776
Cash and cash equivalents at end of period	$ 268,229	$ 245,980