Share-based compensation plans (Details 4) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Y	Dec. 31, 2010	Dec. 31, 2009
Share-based compensation
Weighted average remaining contractual life of exercisable options (in years)	4.28
Total intrinsic value of options exercised	$ 4.2	$ 3.0	$ 0.1
Closing stock price	$ 35.23
Aggregate intrinsic value of options outstanding	10.8	4.8	10.9
Aggregate intrinsic value of options exercisable	$ 8.4	$ 2.1	$ 1.9
$10.42 - $50.99
Share-based compensation
Exercise price, low end of range (in dollars per share)	$ 10.42
Exercise price, high end of range (in dollars per share)	$ 50.99
Number outstanding (in shares)	2,559,272
Weighted Average Remaining Contractual Life of options outstanding (in years)	5.12
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 33.41
Number Exercisable (in shares)	2,079,953
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 34.02
$10.42 - $25.01
Share-based compensation
Exercise price, low end of range (in dollars per share)	$ 10.42
Exercise price, high end of range (in dollars per share)	$ 25.01
Number outstanding (in shares)	452,162
Weighted Average Remaining Contractual Life of options outstanding (in years)	5.80
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 22.73
Number Exercisable (in shares)	343,009
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 22.57
$25.05 - $28.50
Share-based compensation
Exercise price, low end of range (in dollars per share)	$ 25.05
Exercise price, high end of range (in dollars per share)	$ 28.50
Number outstanding (in shares)	189,500
Weighted Average Remaining Contractual Life of options outstanding (in years)	6.63
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 25.45
Number Exercisable (in shares)	168,500
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 25.31
$28.95 - $28.95
Share-based compensation
Exercise price, low end of range (in dollars per share)	$ 28.95
Exercise price, high end of range (in dollars per share)	$ 28.95
Number outstanding (in shares)	265,447
Weighted Average Remaining Contractual Life of options outstanding (in years)	5.57
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 28.95
Number Exercisable (in shares)	267,447
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 28.95
$29.17 - $32.77
Share-based compensation
Exercise price, low end of range (in dollars per share)	$ 29.17
Exercise price, high end of range (in dollars per share)	$ 32.77
Number outstanding (in shares)	265,709
Weighted Average Remaining Contractual Life of options outstanding (in years)	5.86
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 30.59
Number Exercisable (in shares)	171,543
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 31.32
$33.00 - $37.36
Share-based compensation
Exercise price, low end of range (in dollars per share)	$ 33.00
Exercise price, high end of range (in dollars per share)	$ 37.36
Number outstanding (in shares)	274,750
Weighted Average Remaining Contractual Life of options outstanding (in years)	8.32
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 34.36
Number Exercisable (in shares)	44,750
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 34.63
$37.76 - $38.00
Share-based compensation
Exercise price, low end of range (in dollars per share)	$ 37.76
Exercise price, high end of range (in dollars per share)	$ 38.11
Number outstanding (in shares)	275,499
Weighted Average Remaining Contractual Life of options outstanding (in years)	1.94
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 37.83
Number Exercisable (in shares)	275,499
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 37.87
$38.11 - $39.24
Share-based compensation
Exercise price, low end of range (in dollars per share)	$ 38.40
Exercise price, high end of range (in dollars per share)	$ 41.33
Number outstanding (in shares)	279,224
Weighted Average Remaining Contractual Life of options outstanding (in years)	3.94
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 38.23
Number Exercisable (in shares)	279,224
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 38.23
$39.94 - $43.04
Share-based compensation
Exercise price, low end of range (in dollars per share)	$ 42.97
Exercise price, high end of range (in dollars per share)	$ 44.97
Number outstanding (in shares)	325,698
Weighted Average Remaining Contractual Life of options outstanding (in years)	3.96
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 41.40
Number Exercisable (in shares)	300,698
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 41.50
$44.87 - $50.50
Share-based compensation
Exercise price, low end of range (in dollars per share)	$ 45.84
Exercise price, high end of range (in dollars per share)	$ 50.50
Number outstanding (in shares)	221,783
Weighted Average Remaining Contractual Life of options outstanding (in years)	4.24
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 45.64
Number Exercisable (in shares)	221,783
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 54.64
$50.99 - $50.99
Share-based compensation
Exercise price, low end of range (in dollars per share)	$ 50.99
Exercise price, high end of range (in dollars per share)	$ 50.99
Number outstanding (in shares)	7,500
Weighted Average Remaining Contractual Life of options outstanding (in years)	5.04
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 50.99
Number Exercisable (in shares)	7,500
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 50.99