Share-based compensation (Details)	12 Months Ended		12 Months Ended
	Dec. 31, 2011 Long-term incentive plan (LTIP)	May 31, 2010 Equity and Incentive Plan (EIP)	Dec. 31, 2011 Equity and Incentive Plan (EIP) Restricted shares installment	Dec. 31, 2011 Equity and Incentive Plan (EIP) Restricted stock units	Dec. 31, 2011 Stock Option and Incentive Plan (SOIP) Y	Dec. 31, 2011 Stock Option and Incentive Plan (SOIP) Nonqualified stock options (NQSOs) Y	Dec. 31, 2011 Stock Option and Incentive Plan (SOIP) Stock appreciation rights (SARs) Y
Share-based compensation
Reduction in shares available for grant due to share awards granted	368,323		72,036	713,144
Reduction in shares available for grant for each share award granted			4	4
Aggregate shares of issuance of common stock approved by shareholders		4,000,000
Cumulative shares granted			18,009	178,286
Outstanding shares					600,000
Percentage of award exercisable in installments (as a percent)						25%	25%
Period during which the award vests and becomes exercisable in installments (in years)						4	4
Period award expires if not exercised (in years)						Ten years	Ten years
The number of equal annual increments for which the awards become unrestricted (in installments)			4
Period of time from grant date after which awards become unrestricted (in years)					4
Performance period (in years)	3 years
Exception to forfeiture, minimum requisite service period (in months)	12 months