Net income per common share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011
Computation of basic and diluted earnings per share [Abstract]
Net income	$ 10,173	$ 77,489	$ 67,699	$ 256,069
Weighted Average Number of Shares Outstanding, Basic	194,958,000	191,640,000	194,157,000	190,854,000
Dilutive effect of stock options and RSUs	3,725,000	4,266,000	3,709,000	4,311,000
Dilutive effect of convertible debt	5,721,774	7,385,008	6,687,078	7,521,453
Weighted average common and common equivalent shares outstanding	204,405,000	203,291,000	204,553,000	202,686,000
Earnings Per Share, Basic	$ 0.05	$ 0.40	$ 0.35	$ 1.34
Earnings Per Share, Diluted	$ 0.05	$ 0.38	$ 0.33	$ 1.26
Weighted average conversion price per share used in calculating dilutive effect of convertible debt (amount per share)	$ 27.21		$ 27.51
Number of antidilutive option shares (in shares)	124,395	62,541	110,021	53,374