Share-based compensation plans (stock options) (Details) (Stock Options [Member], USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period, minimum (in years)	1
Vesting period, maximum (in years)	3
Term of award from grant date to expiration date (in years)	7
Exercise period before cancellation of award following voluntary termination of employment (in months)	3M
Black-Scholes option pricing model assumptions [Abstract]
Risk-free interest rate (in hundredths)	2.00%	2.20%	1.60%
Dividend yield (in hundredths)	3.90%	3.20%	3.40%
Expected volatility (in hundredths)	58.60%	54.00%	44.20%
Weighted-average option life (in years)	4.3	4.8	4.6
Shares of common stock into which each option is convertible (in shares)	1
Options outstanding [rollforward]
Outstanding, beginning of year (in options)	2,913,000	2,841,000	3,105,000
Granted (in options)	598,000	695,000	790,000
Exercised (in options)	(383,000)	(185,000)	(3,000)
Forfeited or expired (in options)	(362,000)	(438,000)	(1,051,000)
Outstanding, end of year (in options)	2,766,000	2,913,000	2,841,000
Exercisable, end of year (in options)	1,685,000	1,675,000	1,643,000
Weighted-average exercise price [Abstract]
Outstanding at beginning of year (in dollars per option)	$ 22.60	$ 24.64	$ 33.50
Granted (in dollars per option)	$ 25.56	$ 18.37	$ 9.75
Exercised (in dollars per option)	$ 16.56	$ 11.08	$ 9.73
Forfeited or expired (in dollars per option)	$ 28.87	$ 33.94	$ 39.68
Outstanding at end of period (in dollars per option)	$ 23.26	$ 22.60	$ 24.64
Exercisable at end of period (in dollars per option)	$ 25.18	$ 27.66	$ 30.60
Aggregate intrinsic value [Abstract]
Aggregate intrinsic value, options outstanding, end of year	$ 7,519
Exercisable at end of period	3,378
Weighted-average remaining contractual term [Abstract]
Weighted-average remaining contractual term, options outstanding, end of year (in years)	3.7
Weighted-average remaining contractual term, options exercisable, end of year (in years)	2.6
Weighted-average grant-date fair value, options granted (in dollars per option)	$ 9.37	$ 6.86	$ 2.82
Intrinsic value, options exercised	$ 3,452	$ 1,834	$ 16