Share-based compensation (Details 4) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Y	Dec. 31, 2010 Y	Dec. 31, 2009 Y
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 (in years)	3
LTIP linked to TRS and other performance conditions (in shares)
Outstanding, beginning of period (in shares)	126,782	36,198
Granted (in shares)	75,015	97,191	36,198
Forfeited (in shares)	(4,412)	(6,607)
Outstanding, end of period (in shares)	197,385	126,782	36,198
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 20.33	$ 14.85
Granted (in dollars per share)	$ 35.46	$ 22.45	$ 14.85
Forfeited (in dollars per share)	$ 29.56	$ 21.53
Outstanding, end of period (in dollars per share)	$ 25.94	$ 20.33	$ 14.85
Total weighted-average grant-date fair value	$ 2.7	$ 2.2	$ 0.5
Risk-free interest rate (as a percent)	1.25%	1.30%	1.30%
Expected life (in years)	3	3	3
Expected volatility (as a percent)	27.80%	27.90%	23.70%
Range of expected volatility for Peer Group, minimum (as a percent)	21.20%	22.30%	20.80%
Range of expected volatility for Peer Group, maximum (as a percent)	82.60%	52.30%	46.90%
Grant date fair value (in dollars per share)	$ 35.46	$ 22.45	$ 14.85
Unrecognized compensation cost	2.4
Weighted-average period over which unrecognized compensation cost expected to be recognized (in years)	1.2
LTIP linked to other performance conditions
LTIP linked to TRS and other performance conditions (in shares)
Outstanding, beginning of period (in shares)	161,310	24,131
Granted (in shares)	113,831	160,939	24,131
Cancelled (in shares)	(81,908)
Forfeited (in shares)	(10,735)	(23,760)
Outstanding, end of period (in shares)	182,498	161,310	24,131
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 18.66	$ 16.99
Granted (in dollars per share)	$ 24.96	$ 18.95	$ 16.99
Cancelled (in dollars per share)	$ 18.38
Forfeited (in dollars per share)	$ 20.12	$ 18.90
Outstanding, end of period (in dollars per share)	$ 22.63	$ 18.66	$ 16.99
Total weighted-average grant-date fair value	3.0	0.4
Unrecognized compensation cost	2.3
Weighted-average period over which unrecognized compensation cost expected to be recognized (in years)	1.6
2009-2011 LTIP linked to TRS
Share-based compensation
Period for calculating performance goals based on average consolidated net income (in years)	3
Period for calculating performance goals based on average net income (in years)	3
2011-2013 LTIP linked to TRS
Share-based compensation
Period for calculating performance goals based on average consolidated net income (in years)	3
Period for calculating performance goals based on average net income (in years)	3
LTIP linked to TRS and other performance conditions (in shares)
Outstanding, end of period (in shares)	75,015
Weighted-average grant-date fair value per share
Outstanding, end of period (in dollars per share)	$ 35.46
Total weighted-average grant-date fair value	2.7
Performance period for calculation of grant date fair values of shares (in years)	3
Historical period for calculation of annual dividend yield (in years)	3
Grant date fair value (in dollars per share)	$ 35.46
2011-2013 LTIP linked to other performance conditions
LTIP linked to TRS and other performance conditions (in shares)
Outstanding, end of period (in shares)	113,831
Weighted-average grant-date fair value per share
Outstanding, end of period (in dollars per share)	$ 24.96
Grant date price, first (in dollars per share)	$ 24.95
Total weighted-average grant-date fair value	$ 2.8
Grant date price, second (in dollars per share)	$ 26.25
2009-2011 LTIP
Share-based compensation
Average period used to calculate performance goals (in years)	2
2010-2012 LTIP
Share-based compensation
Average period used to calculate performance goals (in years)	2