Long-term incentive plan (Details) (Long-term incentive plan, USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012 Y	Dec. 31, 2011 Y
Units
Outstanding at the beginning of the period (in shares)	485,781
Granted (in shares)	209,009
Additional shares from dividends	8,172
Vested (in shares)	(231,687)
Outstanding at the end of the period (in shares)	471,275	485,781
Grant Date Weighted Average Fair Value per Unit
Outstanding at the beginning of the period (in dollars per share)	$ 11.49
Granted (in dollars per share)	$ 14.65
Additional shares from dividends (in dollars per share)	$ 12.02
Vested (in dollars per share)	$ 10.10
Outstanding at the end of the period (in dollars per share)	$ 13.81	$ 11.49
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model
Weighted average risk free rate of return, minimum (as a percent)	0.19%	0.15%
Weighted average risk free rate of return, maximum (as a percent)	0.51%	0.28%
Dividend yield (as a percent)	8.30%	7.90%
Expected volatility Company (as a percent)	22.20%	22.20%
Weighted average remaining measurement period (in years)	1.92	0.87
Minimum
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model
Expected volatility of peer companies (as a percent)	17.10%	17.30%
Maximum
Assumptions for calculation of simulated total shareholder return under the Monte Carlo model
Expected volatility of peer companies (as a percent)	112.80%	112.90%