Shareholders' equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Shareholders' equity [Abstract]
Sale of common stock (in shares)		74,883,720
Sale of common stock, price per share (in dollars per share)		$ 10.75
Sale of common stock, net proceeds	$ 0	$ 772,376	$ 0
Shareholder Rights Agreement [Abstract]
Shareholder rights accompanying each outstanding share of the company's common stock (in number of Rights)	1
Distribution Date, description	the earlier of ten days after a public announcement that a person has become an Acquiring Person, or ten business days after a person announces or begins a tender offer in which consummation of such offer would result in a person becoming an Acquiring Person
Common stock, beneficial ownership threshold to be considered an Acquiring Person (in hundredths)	5.00%
Common shares purchaseable per Right (in shares)	0.5
Purchase price (in dollars per share)	$ 25
Purchase price (in dollars per one-half share)	$ 12.50
Redemption price (in dollars per Right)	$ 0.001
Convertible Junior Subordinated Debentures Due 2063 [Member]
Debt Instrument [Line Items]
Shares reserved for conversion under convertible debt (in shares)	28,900,000
Convertible Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Shares reserved for conversion under convertible debt (in shares)	25,700,000