Net income per common share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
Earnings Per Share [Abstract]
Net income	$ 111,495	$ 105,401	$ 99,806	$ 78,579	$ 59,690	$ 10,173	$ (21,184)	$ 78,710	$ 395,281	$ 127,389	$ 336,705
Weighted average common shares outstanding									198,291,000	194,595,000	191,283,000
Dilutive effect of stock options and RSUs									3,910,000	3,650,000	4,207,000
Dilutive effect of convertible debt									15,429,003	7,531,111	8,029,255
Weighted average common and potential common shares outstanding									217,630,000	205,776,000	203,519,000
Basic net income per common share									$ 1.99	$ 0.65	$ 1.76
Diluted net income per common share	$ 0.50	$ 0.48	$ 0.46	$ 0.37	$ 0.28	$ 0.05	$ (0.10)	$ 0.39	$ 1.82	$ 0.62	$ 1.65
Weighted average conversion price per share used in calculating dilutive effect of convertible debt (amount per share)									$ 26.32	$ 27.36	$ 28.50
Number of antidilutive option shares (in shares)									0	98,068	53,374