Marketable Securities - Schedule of Unrealized Losses by Investment Category (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Fair value of less than 12 months	$ 172,355	$ 38,753
Unrealized losses of less than 12 months	(1,576)	(75)
Fair value of 12 months or greater	373
Unrealized losses of 12 months or greater	(3)
Fair value, Total	172,728	38,753
Unrealized losses, Total	(1,579)	(75)
U.S. government and agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value of less than 12 months	39,004	4,502
Unrealized losses of less than 12 months	(249)	(1)
Fair value of 12 months or greater
Unrealized losses of 12 months or greater
Fair value, Total	39,004	4,502
Unrealized losses, Total	(249)	(1)
Corporate debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value of less than 12 months	70,496	24,203
Unrealized losses of less than 12 months	(648)	(30)
Fair value of 12 months or greater
Unrealized losses of 12 months or greater
Fair value, Total	70,496	24,203
Unrealized losses, Total	(648)	(30)
Mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value of less than 12 months	33,607	10,048
Unrealized losses of less than 12 months	(463)	(44)
Fair value of 12 months or greater	373
Unrealized losses of 12 months or greater	(3)
Fair value, Total	33,980	10,048
Unrealized losses, Total	(466)	(44)
Municipal bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value of less than 12 months	28,653
Unrealized losses of less than 12 months	(211)
Fair value of 12 months or greater
Unrealized losses of 12 months or greater
Fair value, Total	28,653
Unrealized losses, Total	(211)
Other debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value of less than 12 months	595
Unrealized losses of less than 12 months	(5)
Fair value of 12 months or greater
Unrealized losses of 12 months or greater
Fair value, Total	595
Unrealized losses, Total	$ (5)