Convertible Debentures and Revolving Credit Facility (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Mar. 31, 2012		Jun. 30, 2012 2.625% Senior Convertible Debentures [Member]	Mar. 31, 2012 2.625% Senior Convertible Debentures [Member]	Jun. 30, 2010 2.625% Senior Convertible Debentures [Member]	Jun. 30, 2012 3.125% Junior Convertible Debentures [Member]	Mar. 31, 2012 3.125% Junior Convertible Debentures [Member]	Mar. 31, 2007 3.125% Junior Convertible Debentures [Member]
Debt Instrument [Line Items]
Principal amount of Debentures				$ 600,000	$ 600,000	$ 600,000	$ 689,635	$ 689,635	$ 1,000,000
Unamortized discount of liability component				(76,425)	(80,311)		(324,273)	(325,448)
Hedge accounting adjustment - sale of interest rate swap				22,085	23,208
Unamortized discount of embedded derivative from date of issuance							(1,431)	(1,446)
Debt Instrument Carrying Amount Of Liability Component							363,931	362,741
Carrying value of embedded derivative component	1,545	931					1,545	931
Net carrying value of Debentures	911,135	906,569	[1]	545,660	542,897		365,476	363,672
Equity component - net carrying value				$ 105,620	$ 105,620		$ 229,513	$ 229,513

[1] Derived from audited financial statements