Net income per common share (Details) (USD $) In Thousands, except Share data	3 Months Ended		6 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Computation of basic and diluted earnings per share [Abstract]
Net income from continuing operations	$ 79,287	$ 104,748	$ 178,580	$ 196,625
Weighted average common shares outstanding	190,809,000	186,303,000	190,461,000	185,922,000
Dilutive effect of stock options and RSUs	4,107,000	4,401,000	4,332,000	4,466,000
Dilutive effect of convertible debt	5,282,795	0	7,589,675	0
Weighted average common and common equivalent shares outstanding	200,199,000	190,704,000	202,383,000	190,388,000
Basic net income per common share	$ 0.42	$ 0.56	$ 0.94	$ 1.06
Diluted net income per common share - continuing operations	$ 0.40	$ 0.55	$ 0.88	$ 1.03
Weighted average conversion price per share used in calculating dilutive effect of convertible debt (amount per share)	$ 28.64		$ 28.78
Diluted net loss per common share - discontinued operations	$ 0.00	$ (0.01)	$ 0.00	$ (0.02)
Number of antidilutive option shares (in shares)	119,838	353,184	53,374	432,134