Net income (loss) per common share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]
Net income (loss)	$ 99,806	$ (21,184)	$ 178,385	$ 57,526
Weighted Average Number of Shares Outstanding, Basic	197,825,000	194,060,000	197,388,000	193,756,000
Dilutive effect of stock options and RSUs	4,144,000	0	3,908,000	3,701,000
Dilutive effect of convertible debt	14,506,100	0	13,075,168	7,169,730
Weighted average common and common equivalent shares outstanding	216,475,000	194,060,000	214,371,000	204,627,000
Earnings Per Share, Basic	$ 0.50	$ (0.11)	$ 0.90	$ 0.30
Earnings Per Share, Diluted	$ 0.46	$ (0.11)	$ 0.83	$ 0.28
Weighted average conversion price per share used in calculating dilutive effect of convertible debt (amount per share)	$ 26.42		$ 26.54	$ 27.65
Number of antidilutive option shares (in shares)		3,367,634		110,021