Net income per common share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income	$ 105,401	$ 10,173	$ 283,786	$ 67,699
Weighted average common shares outstanding	198,759,000	194,958,000	197,845,000	194,157,000
Dilutive effect of stock options and RSUs	3,834,000	3,725,000	3,882,000	3,709,000
Dilutive effect of convertible debt	16,496,384	5,721,774	14,215,573	6,687,078
Weighted average common and potential common shares outstanding	219,089,000	204,405,000	215,943,000	204,553,000
Basic net income per common share	$ 0.53	$ 0.05	$ 1.43	$ 0.35
Diluted net income per common share	$ 0.48	$ 0.05	$ 1.31	$ 0.33
Weighted average conversion price per share used in calculating dilutive effect of convertible debt (amount per share)	$ 26.19	$ 27.21	$ 26.43	$ 27.51
Number of antidilutive option shares (in shares)		124,395		110,021