Earnings per share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2010
Share-based Compensation [Abstract]
Net Income (Loss) Attributable to Parent	$ 30,344	$ 27,560	$ 27,560
Net Income (Loss) Available to Common Stockholders, Basic	$ 30,344	$ 27,560
Weighted average number of common shares outstanding - basic	109,211	108,769
Earnings Per Share, Basic	$ 0.28	$ 0.25
Incremental Common Shares Attributable to Share-based Payment Arrangements	0	1
Weighted Average Number of Shares Outstanding, Diluted	109,211	108,770
Earnings Per Share, Diluted	$ 0.28	$ 0.25