Net income per common share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]
Net income	$ 89,909	$ 78,579
Weighted average common shares outstanding	200,187,000	196,950,000
Dilutive effect of stock options and RSUs	3,792,000	3,672,000
Dilutive effect of convertible debt	20,547,832	11,644,236
Weighted average common and potential common shares outstanding	224,527,000	212,266,000
Basic net income per common share	$ 0.45	$ 0.40
Diluted net income per common share	$ 0.40	$ 0.37
Weighted average conversion price per share used in calculating dilutive effect of convertible debt (amount per share)	$ 25.75	$ 26.66
Number of antidilutive option shares (in shares)	0	50,198