Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Operating activities:
Net income	$ 112,170	$ 52,116	$ 38,147
Adjustments required to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	11,005	6,202	3,277
Bad debt expense	38	159	42
Stock-based compensation	40,353	32,866	29,053
Write-down of inventory and other	3,946	3,085	2,094
Excess tax benefit from stock-based awards		(12,038)	(3,567)
Deferred tax provision	42,624	25,803	22,296
Change in fair value of contingent consideration	1,337	1,590	1,053
Changes in assets and liabilities:
Accounts receivable	(15,289)	(11,550)	(10,930)
Inventories	(15,686)	(12,284)	(11,473)
Prepaid expenses and other assets	(4,466)	(2,366)	(2,290)
Accounts payable	4,412	7,565	(2,645)
Accrued expenses and other liabilities	7,722	22,223	10,020
Deferred revenue	4,380	1,745	1,718
Net cash provided by operating activities	192,546	115,116	76,795
Investing activities:
Purchases of marketable securities	(325,408)	(278,501)	(260,975)
Proceeds from the sale and maturity of marketable securities	206,909	205,482	219,639
Purchase of other investment	(6,400)	(2,899)	(750)
Purchases of property and equipment	(55,863)	(50,415)	(15,624)
Net cash used for investing activities	(180,762)	(126,333)	(57,710)
Financing activities:
Proceeds from the exercise of stock options	9,303	10,660	9,771
Excess tax benefit from stock-based awards		12,038	3,567
Taxes paid related to net share settlement upon vesting of stock awards	(20,317)	(20,105)	(7,313)
Proceeds from the issuance of stock under employee stock purchase plan	2,394	1,720	1,135
Principal payments on capital lease obligation	(517)	(446)
Net cash (used for) provided by financing activities	(9,137)	3,867	7,160
Effect of exchange rate changes on cash	1,288	(1,841)	(415)
Net increase in cash and cash equivalents	3,935	(9,191)	25,830
Cash and cash equivalents at beginning of year	39,040	48,231	22,401
Cash and cash equivalents at end of year	42,975	39,040	48,231
Supplemental disclosure of cash flow information:
Cash paid for income taxes	4,641	1,405	848
Cash paid for interest on capital lease obligation	302	354
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment under capital lease obligation		16,784
Property and equipment in accounts payable and accrued expenses	$ 3,338	$ 5,692	$ 1,797