Net income per common share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2010
Computation of basic and diluted earnings per share [Abstract]
Net income from continuing operations	$ 130,612	$ 101,930	$ 104,748	$ 91,877	$ 336,705	$ 429,167	$ 217,005
Weighted average common shares outstanding					191,283,000	187,066,000	183,642,000
Dilutive effect of stock options and RSUs					4,207,000	4,463,000	3,697,000
Dilutive effect of convertible debt					8,029,255	3,185,591	0
Weighted average common and common equivalent shares outstanding					203,519,000	194,715,000	187,339,000
Basic net income per common share					$ 1.76	$ 2.29	$ 1.18
Diluted net income per common share - continuing operations	$ 0.65	$ 0.52	$ 0.55	$ 0.48	$ 1.65	$ 2.20	$ 1.16
Weighted average conversion price per share used in calculating dilutive effect of convertible debt (amount per share)					$ 28.50	$ 29.61	$ 31.16
Diluted net loss per common share - discontinued operations	$ (0.03)	$ (0.01)	$ (0.01)	$ (0.01)	$ 0.00	$ (0.05)	$ 0.00
Number of antidilutive option shares (in shares)					53,374	132,084	4,109,841