Share-based compensation plans (Details) (USD $) In Millions, except Share data, unless otherwise specified		12 Months Ended				12 Months Ended			12 Months Ended			1 Months Ended				9 Months Ended	12 Months Ended			1 Months Ended		12 Months Ended							12 Months Ended
	Dec. 31, 2011 plan	Dec. 31, 2011 2004 LTIP Plan	Jun. 19, 2008 2004 LTIP Plan	Jun. 20, 2007 2004 LTIP Plan	Jun. 29, 2004 2004 LTIP Plan	Dec. 31, 2011 2004 LTIP Plan Employees	Jun. 19, 2008 2004 LTIP Plan Non-employee directors	Jun. 29, 2004 2004 LTIP Plan Non-employee directors	Dec. 31, 2011 2004 LTIP Plan Non-employee directors Maximum	Dec. 31, 2011 2004 LTIP Plan Non-employee directors Minimum	Dec. 31, 2011 Staff Share Option Plan	Dec. 31, 2007 Performance Accelerated Stock Option Inducement Grants Key executives Breg	Nov. 30, 2007 Performance Accelerated Stock Option Inducement Grants Key executives Breg Y	Dec. 31, 2003 Performance Accelerated Stock Option Inducement Grants Key executives Breg executive	Dec. 31, 2011 Performance Accelerated Stock Option Inducement Grants Key executives Breg	Dec. 31, 2010 Performance Accelerated Stock Option Inducement Grants Bradley Mason Breg	Dec. 31, 2012 Performance Accelerated Stock Option Inducement Grants Bradley Mason Breg	Dec. 31, 2011 Performance Accelerated Stock Option Inducement Grants Bradley Mason Breg	Dec. 31, 2011 Performance Accelerated Stock Option Inducement Grants William Hopson Breg	May 31, 2010 Stock Purchase Plan	Jun. 30, 2008 Stock Purchase Plan	Dec. 31, 2011 Stock Purchase Plan	Jun. 30, 2009 Stock Purchase Plan	Jun. 20, 2008 Stock Purchase Plan	Dec. 31, 2011 Stock Purchase Plan Maximum	Dec. 31, 2011 Stock Purchase Plan Maximum North America, South America and Asia	Dec. 31, 2011 Stock Purchase Plan Maximum Europe	Dec. 31, 2011 Stock Purchase Plan Minimum	Dec. 31, 2011 Stock options	Dec. 31, 2010 Stock options
Share-based compensation plans
Number of stock option and award plans	3
Number of stock purchase plans	1
Share-based compensation
Amount of shares reserved for issuance			3,100,000	2,800,000	2,000,000															1,850,000			1,400,000	950,000
Award vesting period (in years)						3 years			5 years	3 years	5 years			4 years
Award contractual term (in years)		P10Y									P10Y
Number of shares approved for annual grant							5,000	3,000
Full value award limit			100,000
Options outstanding (in shares)		2,371,147									58,125				130,000														2,559,272	2,989,469
Options exercisable (in shares)		1,891,828									58,125				130,000														2,079,953
Restricted stock outstanding (in shares)		69,000
Number of executives to whom awards were granted														2
Exercise price (in dollars per share)														$ 38.00															$ 33.08
Vesting percentage of share-based awards												100.00%
Term of extension of employment agreement by executives of acquired entity (in years)													1
Options exercisable during period (in shares)																50,000	50,000	50,000	50,000
Increase in number of shares authorized																				450,000	500,000
Percentage of compensation of eligible employees to be deducted for purchase of common stock																									25.00%	25.00%	15.00%	1.00%
Purchase price of shares equivalent to fair market value (as a percent)																						85.00%
Cumulative number of shares issued under Employee Stock Purchase Plan																						1,224,756
Grant (in shares)																													386,000
Unamortized compensation expense																													$ 3.4