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
Earnings per share - basic
Net income	$ 41,503	$ 36,735	$ 127,845	$ 104,764
Income allocated to participating securities	(289)	(240)	(865)	(565)
Income available to common shareholders	41,214	36,495	126,980	104,199
Weighted-average shares outstanding	50,680	50,889	50,779	51,112
Earnings per share - basic	$ 0.81	$ 0.72	$ 2.50	$ 2.04
Earnings per share - diluted
Net income	41,503	36,735	127,845	104,764
Income allocated to participating securities	(287)	(166)	(861)	(325)
Re-measurement of share-based awards classified as liabilities	67	(77)	102	(65)
Income available to common shareholders	$ 41,283	$ 36,492	$ 127,086	$ 104,374
Weighted-average shares outstanding	50,680	50,889	50,779	51,112
Dilutive impact of potential common shares	349	314	299	431
Weighted-average shares and potential common shares outstanding	51,029	51,203	51,078	51,543
Earnings per share - diluted	$ 0.81	$ 0.71	$ 2.49	$ 2.02
Antidilutive options excluded from calculation	1,118	1,961	1,118	1,961