Earnings per share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Share-based Compensation [Abstract]
Net Income (Loss) Attributable to Parent	$ 34,953	$ 31,737	$ 95,882	$ 89,060
Net Income (Loss) Available to Common Stockholders, Basic	$ 34,953	$ 31,737	$ 95,882	$ 89,060
Weighted average number of common shares outstanding - basic	109,645	109,245	109,473	109,052
Earnings Per Share, Basic	$ 0.32	$ 0.29	$ 0.88	$ 0.82
Weighted Average Number of Shares Outstanding, Diluted	109,645	109,246	109,473	109,053
Earnings Per Share, Diluted	$ 0.32	$ 0.29	$ 0.88	$ 0.82