Share-based compensation plans (Details 2) (USD $)	3 Months Ended		9 Months Ended				1 Months Ended	5 Months Ended			1 Months Ended		5 Months Ended		1 Months Ended		1 Months Ended		1 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Stock appreciation rights	Dec. 31, 2011 Stock appreciation rights	Mar. 31, 2012 Stock option plans Y	Jun. 01, 2012 Stock option plans Y	Jun. 01, 2012 Stock option plans 2009 share award and incentive plan Class A common shares vesting on June 7, 2015	Jun. 07, 2012 Stock option plans 2009 share award and incentive plan Class A common shares vesting on June 7, 2015	Mar. 31, 2012 Stock option plans 2009 share award and incentive plan Common Class A vesting on March 9, 2015	Mar. 09, 2012 Stock option plans 2009 share award and incentive plan Common Class A vesting on March 9, 2015	Jun. 01, 2012 Deferred shares without performance criteria 2009 share award and incentive plan Class A common shares vesting on June 7, 2015	Jun. 07, 2012 Deferred shares without performance criteria 2009 share award and incentive plan Class A common shares vesting on June 7, 2015	Mar. 31, 2012 Deferred shares without performance criteria 2009 share award and incentive plan Class A common shares vesting on January 30, 2015	Mar. 09, 2012 Deferred shares without performance criteria 2009 share award and incentive plan Class A common shares vesting on January 30, 2015	Mar. 31, 2012 Deferred shares without performance criteria 2009 share award and incentive plan Common Class A vesting on March 9, 2015	Mar. 09, 2012 Deferred shares without performance criteria 2009 share award and incentive plan Common Class A vesting on March 9, 2015	Mar. 31, 2012 Deferred shares with performance criteria Y	Mar. 31, 2012 Deferred shares with performance criteria 2009 share award and incentive plan Common Class A vesting on March 9, 2015	Mar. 09, 2012 Deferred shares with performance criteria 2009 share award and incentive plan Common Class A vesting on March 9, 2015
Shares
Granted (in shares)									330,200		31,700		109,000		3,300		104,000			386,600
Grants in period exercise price (in dollars per share)										$ 8.42		$ 9.95		$ 0.01		$ 0.01		$ 0.01			$ 0.01
Estimated fair value assumptions
Expected share price volatility (as a percent)							58.00%	58.00%											59.00%
Risk-free interest rate (as a percent)							1.69%	1.07%											0.47%
Expected annual dividends per share (in dollars per share)							$ 0	$ 0											$ 0
Expected life of share options/awards (in years)							8	8											3
Additional disclosures
Fair value of liability					$ 98,000	$ 111,000
Fair value of awards issued during the period	$ 0.00	$ 0	$ 5,223,000	$ 5,784,000