Share-based compensation plans (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended	12 Months Ended									12 Months Ended								12 Months Ended		0 Months Ended				0 Months Ended								12 Months Ended			0 Months Ended
	Dec. 31, 2013 share_based_plan	Dec. 31, 2013 share_based_plan	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Share options Class A common shares	Dec. 31, 2013 Stock appreciation rights Class A common shares	Dec. 31, 2013 2000 and 2004 stock option plans Share options Class A common shares	Dec. 31, 2012 2000 and 2004 stock option plans Share options Class A common shares	Dec. 31, 2013 2000 stock option plan Share options Class A common shares	Dec. 31, 2013 2004 stock option plan Share options Class A common shares	Dec. 31, 2013 2007 performance share plan Performance shares Class A common shares	Dec. 31, 2012 2007 performance share plan Performance shares Class A common shares	Dec. 31, 2013 2007 stock appreciation rights plan Stock appreciation rights Class A common shares	Dec. 31, 2012 2007 stock appreciation rights plan Stock appreciation rights Class A common shares	Dec. 31, 2011 2007 stock appreciation rights plan Stock appreciation rights Class A common shares	Dec. 31, 2012 2009 share award and incentive plan Class A common shares	Dec. 31, 2010 2009 share award and incentive plan Class A common shares	Jun. 05, 2009 2009 share award and incentive plan Class A common shares	Dec. 31, 2013 2009 share award and incentive plan Share options Class A common shares	Dec. 31, 2012 2009 share award and incentive plan Share options Class A common shares	Feb. 21, 2013 2009 share award and incentive plan Share options Class A common shares vesting in March 2015	Jun. 10, 2013 2009 share award and incentive plan Share options Class A common shares vesting on 10 June 2016	Dec. 12, 2013 2009 share award and incentive plan Share options Class A common shares vesting on 12 December 2016	Dec. 31, 2013 2009 share award and incentive plan Deferred shares without performance criteria Class A common shares	Feb. 21, 2013 2009 share award and incentive plan Deferred shares without performance criteria Class A common shares vesting in March 2015	Mar. 15, 2013 2009 share award and incentive plan Deferred shares without performance criteria Class A common shares vesting in March 2016	Apr. 22, 2013 2009 share award and incentive plan Deferred shares without performance criteria Class A common shares vesting in July 2013	Jun. 10, 2013 2009 share award and incentive plan Deferred shares without performance criteria Class A common shares vesting on 10 June 2016	Jun. 28, 2013 2009 share award and incentive plan Deferred shares without performance criteria Class A common shares vesting on 28 June 2016	Aug. 14, 2013 2009 share award and incentive plan Deferred shares without performance criteria Class A common shares vesting on 14 August 2016	Jun. 28, 2013 2009 share award and incentive plan Restricted shares without performance criteria Class A common shares vesting on 28 June 2016	Jun. 28, 2013 2009 share award and incentive plan Restricted shares without performance criteria Class A common shares vesting on the earlier of 28 June 2014 or the day before the 2014 annual general meeting of the Company	Dec. 31, 2013 2009 share award and incentive plan Deferred compensation Class A common shares	Dec. 31, 2012 2009 share award and incentive plan Deferred compensation Class A common shares	Dec. 31, 2013 2009 share award and incentive plan Deferred shares with performance criteria Class A common shares	Feb. 21, 2013 2009 share award and incentive plan Deferred shares with performance criteria Class A common shares vesting in March 2015	Mar. 15, 2013 2009 share award and incentive plan Deferred shares with performance criteria Class A common shares vesting in March 2016
Share-based compensation plans
Number of share-based compensation plans	4	4
Share-based compensation arrangement by share-based payment award, number of share-based plans expired	1	1
Compensation cost		$ 10,388	$ 6,834	$ 6,752
Cash received from exercised options and vested awards		7	3	3
Total unrecognized compensation cost related to unexercised stock options and unvested share awards	10,884	10,884
Total unrecognized compensation cost related to unexercised stock options and unvested share awards, recognition period (in months)	28 months
Award expiration period					10 years	3 years	10 years
Maximum number of options to purchase class A common shares under the plan (in shares)									750,000	1,000,000	500,000
Award vesting period							3 years
Number of shares reserved for issuance									23,500	453,950								1,084,550						800,400									1,481,827		681,427
Number of shares available for grant							0				0
Fair value of options exercised							161												1,025
Share-based compensation other than options granted in period (in shares)											0	0	0	0	0										6,600	114,000	29,370	34,400	33,200	40,000	33,200	12,400	618,170	1,042,900		24,600	290,400
Increase in number of shares authorized (in shares)																5,000,000	4,000,000
Options granted in period (in shares)							0	0											756,650	811,450	31,700	289,000	435,950
Options granted in period, weighted average exercise price (in dollars per share)							$ 0.00	$ 0.00											$ 13.26	$ 10.09	$ 9.95	$ 11.74	$ 14.51
Deferred shares purchase price (in dollars per share)											$ 0.01														$ 0.01	$ 0.01	$ 0.01	$ 0.01	$ 0.01	$ 0.01	$ 0.01	$ 0.01				$ 0.01	$ 0.01
Total weighted-average fair value of stock options granted																			5,048
Fair value of options exercisable, total																			3,938
Number of options vested during the period																			696,850
Fair value of awards issued during the period																																	4,227
Fair value of shares vested in the period																																	$ 3,193
Options, vested and expected to vest, outstanding, number																																	0