Share-based compensation (Details 3) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 Y	Mar. 31, 2011
Restricted shares
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	46,807	89,709
Vested (in shares)	(8,700)
Forfeited (in shares)		(1,000)
Outstanding, end of period (in shares)	38,107	88,709
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 24.45	$ 24.64
Vested (in dollars per share)	$ 27.17
Forfeited (in dollars per share)		$ 25.36
Outstanding, end of period (in dollars per share)	$ 23.83	$ 24.63
Unrecognized compensation cost	$ 0.2
Weighted-average period over which unrecognized compensation cost expected to be recognized (in years)	2.6
Aggregate fair value of vested shares	0.2
Tax benefits relating to vesting of share-based awards	0.1
Restricted stock units
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	247,286	146,500
Granted (in shares)	92,512	85,017
Vested (in shares)	(21,247)
Forfeited (in shares)		(1,000)
Outstanding, end of period (in shares)	318,551	230,517
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 21.80	$ 19.80
Granted (in dollars per share)	$ 25.98	$ 24.95
Vested (in dollars per share)	$ 24.95
Forfeited (in dollars per share)		$ 22.60
Outstanding, end of period (in dollars per share)	$ 22.80	$ 21.69
Total weighted-average grant-date fair value	2.4	2.1
Unrecognized compensation cost	4.8
Weighted-average period over which unrecognized compensation cost expected to be recognized (in years)	3.1
Aggregate fair value of vested shares and related dividends	0.6
Tax benefits	$ 0.2