Earnings per share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Earnings Per Share [Abstract]
Net income (loss)	$ (12,277)		$ 23,986		$ (16,678)		$ 49,032
Amount allocated to common shareholders (in hundredths)	100.00%		91.97%		100.00%		91.91%
Net income (loss) allocated to common shareholders	(12,277)		22,061		(16,678)		45,066
Basic EPS:
Income (loss) allocable to common shareholders - in Shares (000)	88,452,000		93,105,000		88,589,000		93,036,000
Income (loss) allocable to common shareholders (in dollars per share)	$ (0.14)		$ 0.24		$ (0.19)		$ 0.48
Effect of Dilutive Securities:
Stock compensation plans	0		0		0		0
Share-based stock awards - in Shares (000)	0		462,000		0		396,000
Diluted EPS:
Income (loss) applicable to common shareholders with assumed conversion	$ (12,277)		$ 22,061		$ (16,678)		$ 45,066
Income (loss) applicable to common shareholders with assumed conversion - in Shares (000)	88,452,000		93,567,000		88,589,000		93,432,000
Income (loss) applicable to common shareholders with assumed conversion (in dollars per share)	$ (0.14)		$ 0.24		$ (0.19)		$ 0.48
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Options to purchase common stock outstanding (in shares)	626,334		641,187		626,334		641,187
Shares repurchased (in shares)					400,000
Weighted Average Number of Shares Outstanding, Basic	395,791		343,148		370,658		280,366
Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted EPS (in shares)	626,334	[1]	349,000	[1]	632,334	[1]	349,000	[1]
Service-Based Share Awards [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted EPS (in shares)	395,791		0		370,658		0

[1] These stock option awards were excluded from the calculation of diluted EPS because the exercise price of these options was greater than the average market price of the underlying common stock, except in periods when we had a net loss where all option were excluded because of their antidilutive effect on earnings per share.