Unobservable Input Reconciliation Assets (Details) (USD $)	6 Months Ended	12 Months Ended
	Sep. 30, 2012	Mar. 31, 2012
Auction rate securities [Member]
Reconciliation for all assets measured at fair value on a recurring basis, excluding accrued interest components, using significant unobservable inputs [Rollforward]
Fair value, measurement with unobservable inputs reconciliation, beginning	$ 10,246,000	$ 12,475,000
Included in earnings	(334,000)	271,000
Purchases, sales, issuances, and settlements, net	24,390,000	(2,500,000)
Fair value, measurement with unobservable inputs reconciliation, ending	34,302,000	10,246,000
Corporate Debt [Member]
Reconciliation for all assets measured at fair value on a recurring basis, excluding accrued interest components, using significant unobservable inputs [Rollforward]
Fair value, measurement with unobservable inputs reconciliation, beginning	4,625,000	3,500,000
Included in earnings	0	0
Purchases, sales, issuances, and settlements, net	1,056,000	1,125,000
Fair value, measurement with unobservable inputs reconciliation, ending	5,681,000	4,625,000
Total Gains (Losses) [Member]
Reconciliation for all assets measured at fair value on a recurring basis, excluding accrued interest components, using significant unobservable inputs [Rollforward]
Fair value, measurement with unobservable inputs reconciliation, beginning	0	0
Included in earnings	(334,000)	271,000
Purchases, sales, issuances, and settlements, net	0	0
Fair value, measurement with unobservable inputs reconciliation, ending		$ 0