Share-based compensation plans - Grants in period and fair value assumptions for share-based compensation plans (Details) (Class A common shares vesting in March 2017, 2009 share award and incentive plan, USD $)	3 Months Ended
	Mar. 31, 2014
Deferred shares without performance criteria
Share-based compensation plans
Granted deferred shares (in shares)	94,000,000
Deferred shares, purchase price (in dollars per share)	$ 10.00
Expected share price volatility	46.00%
Risk-free interest rate	0.91%
Expected dividends per share	$ 0
Expected life of awards	3 years
Deferred shares with performance criteria
Share-based compensation plans
Granted deferred shares (in shares)	251,400,000
Deferred shares, purchase price (in dollars per share)	$ 10.00
Expected share price volatility	46.00%
Risk-free interest rate	0.89%
Expected dividends per share	$ 0
Expected life of awards	3 years