Net (loss) income per common share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Computation of basic and diluted earnings per share [Abstract]
Net (loss) income	$ (21,184)	$ 79,287	$ 57,526	$ 178,580
Weighted Average Number of Shares Outstanding, Basic	194,060,000	190,809,000	193,756,000	190,461,000
Dilutive effect of stock options and RSUs	0	4,107,000	3,701,000	4,332,000
Dilutive effect of convertible debt	0	5,282,795	7,169,730	7,589,675
Weighted average common and common equivalent shares outstanding	194,060,000	200,199,000	204,627,000	202,383,000
Earnings Per Share, Basic	$ (0.11)	$ 0.42	$ 0.30	$ 0.94
Earnings Per Share, Diluted	$ (0.11)	$ 0.40	$ 0.28	$ 0.88
Weighted average conversion price per share used in calculating dilutive effect of convertible debt (amount per share)	$ 27.51		$ 27.65
Number of antidilutive option shares (in shares)	3,367,634	119,838	110,021	53,374