Shareholders' equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Shares outstanding [Abstract]
Common stock shares outstanding, beginning of year (in shares)	51,338,000
Balance, end of year (in shares)	50,826,000	51,338,000
Share repurchases [Abstract]
Payments for common shares repurchased	$ 23,620	$ 2,999	$ 1,319
Common Stock [Member]
Shares outstanding [Abstract]
Common stock shares outstanding, beginning of year (in shares)	51,338,000	51,189,000	51,131,000
Issued (in shares)	499,000	410,000	237,000
Repurchased (in shares)	(940,000)	(167,000)	(120,000)
Retired (in shares)	(71,000)	(94,000)	(59,000)
Balance, end of year (in shares)	50,826,000	51,338,000	51,189,000
Share repurchases [Abstract]
Common shares authorized for repurchase (in shares)	10,000,000
Common shares that remain available for repurchase (in shares)	5,257,000
Common shares repurchased during the period (in shares)	940,000	167,000	120,000
Payments for common shares repurchased	$ 23,620	$ 2,999	$ 1,319
Common stock purchase rights [Abstract]
Number of common shares each right entitles holder to purchase (in shares)	1
Exercise price of common stock purchase rights (in dollars per share)	$ 100
Beneficial ownership that could trigger the exercise of stock purchase rights (in hundredths)	20.00%
Percentage of market price of one share of common stock used to determine number of shares purchasable (in hundredths)	50.00%
Expiration date of common stock purchase rights	December 31, 2016
Redemption price of common stock purchase rights (in dollars per right)	$ 0.01
Maximum allowable period between independent director reviews of the plan (in years)	3Y