Earnings per share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings per share - basic: [Abstract]
Income from continuing operations	$ 144,595	$ 153,395	$ 99,365
Income allocated to participating securities	(823)	(804)	(751)
Income available to common shareholders	143,772	152,591	98,614
Weighted-average shares outstanding (in shares)	51,036	51,123	50,837
Earnings per share - basic (in dollars per share)	$ 2.82	$ 2.98	$ 1.94
Income from continuing operations	144,595	153,395	99,365
Income allocated to participating securities	(586)	(802)	(751)
Re-measurement of share-based awards classified as liabilities	(20)	79	(18)
Income available to common shareholders	$ 143,989	$ 152,672	$ 98,596
Weighted-average shares outstanding (in shares)	51,036	51,123	50,837
Dilutive impact of potential common shares (in shares)	379	202	88
Weighted-average shares and potential common shares outstanding (in shares)	51,415	51,325	50,925
Earnings per share - diluted (in dollars per share)	$ 2.80	$ 2.97	$ 1.94
Antidilutive options excluded from calculation (in shares)	1,835	2,324	2,128