CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (USD $) In Thousands, except Share data, unless otherwise specified	Total	Non-voting Common Stock	Ordinary Shares	Additional Paid-In Capital	Accumulated Other Comprehensive Loss (Income)	Accumulated Deficit	Treasury Stock
BALANCE at Mar. 31, 2010	$ 412,616	$ 4	$ 1,047	$ 910,326	$ (3,392)	$ (365,688)	$ (129,681)
BALANCE (in shares) at Mar. 31, 2010		382,632	104,815,328				(9,945,265)
Increase (Decrease) in Shareholders' Equity
Issuance of common stock under employee stock plans	6,158		8	6,150
Issuance of common stock under employee stock plans (in shares)			956,179
Receipt of Alkermes' stock for the purchase of stock options or to satisfy minimum tax withholding obligations related to stock based awards	(1,414)						(1,414)
Receipt of Alkermes' stock for the purchase of stock options or to satisfy minimum tax withholding obligations related to stock based awards (in shares)							(123,943)
Share-based compensation expense	19,819			19,819
Unrealized gains on marketable securities, net of tax of $372 and $225 for the year ended March 31, 2012 and 2011, respectively	379				379
Net income (loss)	(45,540)					(45,540)
BALANCE at Mar. 31, 2011	392,018	4	1,055	936,295	(3,013)	(411,228)	(131,095)
BALANCE (in shares) at Mar. 31, 2011		382,632	105,771,507				(10,069,208)
Increase (Decrease) in Shareholders' Equity
Issuance of common stock to Elan Corporation, plc in connection with the purchase of Elan Drug Technologies	525,074		319	524,755
Issuance of common stock to Elan Corporation, plc in connection with the purchase of Elan Drug Technologies (in shares)			31,900,000
Issuance of common stock under employee stock plans	20,864		24	20,840
Issuance of common stock under employee stock plans (in shares)			2,398,422
Receipt of Alkermes' stock for the purchase of stock options or to satisfy minimum tax withholding obligations related to stock based awards	(3,676)						(3,676)
Receipt of Alkermes' stock for the purchase of stock options or to satisfy minimum tax withholding obligations related to stock based awards (in shares)							(205,901)
Share-based compensation expense	28,615			28,615
Excess tax benefit from share-based compensation	4,335			4,335
Conversion of non-voting common stock to common stock		(4)	4
Conversion of non-voting common stock to common stock (in shares)		(382,632)	382,632
Cancellation of treasury stock			(102)	(134,098)			134,200
Cancellation of treasury stock (in shares)			(10,240,031)				10,240,031
Unrealized gains on marketable securities, net of tax of $372 and $225 for the year ended March 31, 2012 and 2011, respectively	627				627
Unrealized loss on cash flow hedge, net of tax of $(194)	(327)				(327)
Net income (loss)	(113,678)					(113,678)
BALANCE at Mar. 31, 2012	853,852		1,300	1,380,742	(2,713)	(524,906)	(571)
BALANCE (in shares) at Mar. 31, 2012			130,212,530				(35,078)
Increase (Decrease) in Shareholders' Equity
Issuance of common stock under employee stock plans	34,360		38	34,322
Issuance of common stock under employee stock plans (in shares)			3,852,577
Receipt of Alkermes' stock for the purchase of stock options or to satisfy minimum tax withholding obligations related to stock based awards	(4,809)						(4,809)
Receipt of Alkermes' stock for the purchase of stock options or to satisfy minimum tax withholding obligations related to stock based awards (in shares)							(278,419)
Share-based compensation expense	34,926			34,926
Excess tax benefit from share-based compensation	8,867			8,867
Unrealized gains on marketable securities, net of tax of $372 and $225 for the year ended March 31, 2012 and 2011, respectively	703				703
Unrealized loss on cash flow hedge, net of tax of $(194)	(72)				(72)
Reclassification of unrealized gains to realized gains	(1,030)				(1,030)
Reclassification of unrealized losses to realized losses	594				594
Net income (loss)	24,983					24,983
BALANCE at Mar. 31, 2013	$ 952,374		$ 1,338	$ 1,458,857	$ (2,518)	$ (499,923)	$ (5,380)
BALANCE (in shares) at Mar. 31, 2013			134,065,107				(313,497)