Share-based compensation (Details 4) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended		12 Months Ended
	Feb. 29, 2012	Mar. 31, 2012 Y	Mar. 31, 2011	Dec. 31, 2011 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)		197,385	126,782	126,782
Granted (in shares)	77,482	77,482	74,540
Vested (in shares)		(35,397)
Forfeited (in shares)			(587)
Outstanding, end of period (in shares)		239,470	200,735	197,385
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)		$ 25.94	$ 20.33	$ 20.33
Granted (in dollars per share)	$ 25.98	$ 30.71	$ 35.46
Vested (in dollars per share)		$ 14.85
Forfeited (in dollars per share)			$ 22.45
Outstanding, end of period (in dollars per share)		$ 29.12	$ 25.94	$ 25.94
Total weighted-average grant-date fair value	$ 2.4		$ 2.6
Grant date price, second (in dollars per share)	$ 30.71
Risk-free interest rate (as a percent)		0.33%		1.25%
Expected life (in years)		3		3
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		$ 35.46
Unrecognized compensation cost		4.0
Weighted-average period over which unrecognized compensation cost expected to be recognized (in years)		1.7
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)		182,498	161,310	161,310
Granted (in shares)	115,104	115,104	113,119
Forfeited (in shares)			(879)
Outstanding, end of period (in shares)		297,602	273,550
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)		$ 22.63	$ 18.66	$ 18.66
Granted (in dollars per share)	$ 25.98	$ 25.98	$ 24.95
Forfeited (in dollars per share)			$ 18.95
Outstanding, end of period (in dollars per share)		$ 23.92	$ 21.26
Total weighted-average grant-date fair value	3.0		2.8
Grant date price, second (in dollars per share)	$ 25.98
Unrecognized compensation cost		$ 4.7
Weighted-average period over which unrecognized compensation cost expected to be recognized (in years)		1.9
2012-2014 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
2010-2012 LTIP
Share-based compensation
Average period used to calculate performance goals (in years)		2