Share-based compensation plans - Options Activity (Detais) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based compensation plans [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	900,000
Share-based Compensation Plans Optoins Outstanding [Roll Forward]
Beginning Balances - shares	2,967,000	3,355,000	3,435,000
Options granted - shares	396,000	290,000	367,000
Options Exercised - shares	(71,000)	(562,000)	(372,000)
Options Forfeited or Expired - shares	(680,000)	(116,000)	(75,000)
Ending Balance - shares	2,612,000	2,967,000	3,355,000
Share-based Compensation Plans Optoins Outstanding Exercise Price [Abstract]
Beginning Balances - Exercise Price	$ 30.92	$ 29.10	$ 28.98
Options granted - Exercise Price	$ 22.55	$ 31.47	$ 19.05
Options Exercised - Exercise Price	$ 19.04	$ 20.66	$ 17.50
Options Forfeited or Expired - Exercise Price	$ 29.92	$ 29.47	$ 31.71
Ending Balance - Exercise Price	$ 30.23	$ 30.92	$ 29.10
Outstanding Options - Remaining Term Years	2 years 5 months 12 days
Outstanding Options - Average Intrinsic Value	$ 5.3
Exercisable Options - Shares	1,974,000	2,400,000	2,600,000
Exercisable Options - Exercise Price	$ 32.15	$ 32.03	$ 30.1
Exercisable Options - Remaining Term Years	1 year 7 months 20 days
Exercisable Options - Average Intrinsic Value	2.2
Options Expected to Vest in Future Periods - Shares	610,000
Options Expected to Vest in Future Periods - Exercise Price	$ 24.37
Options Expected to Vest in Future Periods - Remaining Term Years	4 years 11 months 2 days
Options Expected to Vest in Future Periods - Average Intrinsic Value	3.0
Market Price per Share	$ 28.53
Total intrinsic value of options exercised	0.6	5.6	2.9
Intrinsic value of options exercised per share	$ 8.07	$ 9.99	$ 7.86
Total fair value of options vested	$ 1.8	$ 3.2	$ 5.1