Earnings per share (Details) (USD $) In Thousands, except Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
BASIC EPS:
Net income available to common shareholders	$ 29,763	$ 31,493	$ 57,324	$ 46,883
Average common shares outstanding	109,138,000	98,968,000	108,954,000	93,400,000
Net income per share basic	$ 0.27	$ 0.32	$ 0.53	$ 0.50
DILUTED EPS:
Net income available to common shareholders	29,763	31,493	57,324	46,883
Add: interest expense on convertible bonds	0		0
Net income loss available to common stockholders diluted	29,763	31,493	57,324	46,883
Average common shares outstanding	109,138,000	98,968,000	108,954,000	93,400,000
Add: Equivalents from stock options and restricted	1,000	1,000	1,000	3,000
Add: Equivalents-convertible bonds	0		0
Average common shares and equivalents outstanding	109,139,000	98,969,000	108,955,000	93,403,000
Net income per common share diluted	$ 0.27	$ 0.32	$ 0.53	$ 0.50
Earnings per share (Textuals) [Abstract]
Antidilutive shares not included in computation of diluted earnings per share	3,400,000	4,600,000
Sale of common stock		17,600,160	0	21,487,860
Average value of common stock sold		$ 19.00
Net proceeds from issuance of common stock		$ 320,100		$ 400,064