Share-based compensation (Details 4) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								0 Months Ended	3 Months Ended		0 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Long-term incentive plan (LTIP)	Mar. 31, 2013 LTIP linked to TRS	Mar. 31, 2012 LTIP linked to TRS	Mar. 31, 2013 2010-2012 LTIP	Mar. 31, 2013 2011-2013 LTIP linked to TRS	Mar. 31, 2013 2012-2014 LTIP linked to TRS	Feb. 04, 2013 2013-2015 LTIP linked to TRS	Mar. 31, 2013 LTIP awards linked to other performance conditions	Mar. 31, 2012 LTIP awards linked to other performance conditions	Feb. 04, 2013 2013-2015 LTIP linked to other performance conditions
Share-based compensation
Payout low end of range (as a percent)			0.00%
Payout high end of range (as a percent)			200.00%
Period for calculating total return to shareholders				3 years
Average period used to calculate performance goals						2 years	3 years	3 years
LTIP linked to TRS and other performance conditions (in shares)
Outstanding, beginning of period (in shares)				239,256	197,385					247,175	182,498
Granted (in shares)				89,533	77,482					118,895	115,104
Vested (in shares)				(87,753)	(35,397)					(18,275)
Forfeited (in shares)				(5,972)						(5,971)
Outstanding, end of period (in shares)				235,064	239,470				89,533	341,824	297,602	118,895
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)				$ 29.12	$ 25.94					$ 25.04	$ 22.63
Granted (in dollars per share)				$ 32.69	$ 30.71				$ 26.89	$ 26.89	$ 25.98	$ 26.89
Vested (in dollars per share)				$ 22.45	$ 14.85					$ 18.95
Forfeited (in dollars per share)				$ 32.96						$ 25.94
Outstanding, end of period (in dollars per share)				$ 32.87	$ 29.12				$ 32.69	$ 26.00	$ 23.92	$ 26.89
Total weighted-average grant-date fair value					$ 2.4				$ 2.9		$ 3.0	$ 3.2
Risk-free interest rate (as a percent)				0.38%	0.33%
Expected life				3 years	3 years
Expected volatility (as a percent)				19.40%	25.30%
Range of expected volatility for Peer Group, minimum (as a percent)				12.40%	15.50%
Range of expected volatility for Peer Group, maximum (as a percent)				25.30%	34.50%
Grant date fair value (in dollars per share)				$ 32.69	$ 30.71
Fair value of vested stock (in dollars)				2.2	0.6					0.6
Tax benefits	0.7	0.6		0.9	0.2					0.2
Unrecognized compensation cost				$ 4.1						$ 5.4
Weighted-average period over which unrecognized compensation cost expected to be recognized				1 year 9 months 18 days						1 year 9 months 18 days