Share-based compensation (Details 4) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Long-term incentive plan (LTIP)
Share-based compensation
Payout low end of range (as a percent)			0.00%
Payout high end of range (as a percent)			200.00%
LTIP linked to TRS
Share-based compensation
Period for calculating total return to shareholders			3 years
LTIP linked to TRS and other performance conditions (in shares)
Outstanding, beginning of period (in shares)	239,407	199,563	197,385	126,782	126,782
Granted (in shares)	1,723		80,647	75,015
Vested (in shares)			(35,397)
Forfeited (in shares)	(2,450)	(1,063)	(3,955)	(3,297)
Outstanding, end of period (in shares)	238,680	198,500	238,680	198,500	197,385
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 29.12	$ 25.99	$ 25.94	$ 20.33	$ 20.33
Granted (in dollars per share)	$ 30.71		$ 30.71	$ 35.46
Vested (in dollars per share)			$ 14.85
Forfeited (in dollars per share)	$ 31.09	$ 30.67	$ 30.82	$ 25.10
Outstanding, end of period (in dollars per share)	$ 29.11	$ 25.97	$ 29.11	$ 25.97	$ 25.94
Total weighted-average grant-date fair value	$ 0.1		$ 2.5	$ 2.7
Grant date price, second (in dollars per share)	$ 30.71
Risk-free interest rate (as a percent)			0.33%		1.25%
Expected life			3 years		3 years
Expected volatility (as a percent)			25.30%		27.80%
Range of expected volatility for Peer Group, minimum (as a percent)			15.50%		21.20%
Range of expected volatility for Peer Group, maximum (as a percent)			34.50%		82.60%
Grant date fair value (in dollars per share)	$ 30.71		$ 30.71		$ 35.46
Unrecognized compensation cost	3.0		3.0
Weighted-average period over which unrecognized compensation cost expected to be recognized			1 year 2 months 12 days
Aggregate fair value of vested shares and related dividends			0.6
Tax benefits			0.2
LTIP linked to other performance conditions
LTIP linked to TRS and other performance conditions (in shares)
Outstanding, beginning of period (in shares)	295,184	185,767	182,498	161,310	161,310
Granted (in shares)	4,148		122,852	113,831
Cancelled (in shares)	(17,911)		(17,911)	(81,908)
Forfeited (in shares)	(3,676)	(1,596)	(9,694)	(9,062)
Outstanding, end of period (in shares)	277,745	184,171	277,745	184,171
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 23.95	$ 22.63	$ 22.63	$ 18.66	$ 18.66
Granted (in dollars per share)	$ 27.30		$ 26.05	$ 24.96
Cancelled (in dollars per share)	$ 18.95		$ 18.95	$ 18.38
Forfeited (in dollars per share)	$ 24.78	$ 22.74	$ 24.44	$ 19.61
Outstanding, end of period (in dollars per share)	$ 24.31	$ 22.63	$ 24.31	$ 22.63
Total weighted-average grant-date fair value	0.1		3.2	2.8
Unrecognized compensation cost	$ 3.5		$ 3.5
Weighted-average period over which unrecognized compensation cost expected to be recognized			1 year 6 months
2011-2013 LTIP linked to TRS
Share-based compensation
Average period used to calculate performance goals			3 years
2010-2012 LTIP
Share-based compensation
Average period used to calculate performance goals			2 years
2012-2014 LTIP linked to TRS
Share-based compensation
Average period used to calculate performance goals			3 years
2012-2014 LTIP linked to TRS | Minimum
Weighted-average grant-date fair value per share
Granted (in dollars per share)	$ 27.22
2012-2014 LTIP linked to TRS | Maximum
Weighted-average grant-date fair value per share
Granted (in dollars per share)	$ 27.35
2012-2014 LTIP linked to other performance conditions | Minimum
Weighted-average grant-date fair value per share
Granted (in dollars per share)	$ 27.22
Grant date price, second (in dollars per share)	$ 27.22
2012-2014 LTIP linked to other performance conditions | Maximum
Weighted-average grant-date fair value per share
Granted (in dollars per share)	$ 27.35
Grant date price, second (in dollars per share)	$ 27.35