Income taxes Income Tax Treatment of Distributions (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Common Stock
Income Tax Treatment of Distributions and Dividends [Line Items]
Ordinary income	85.90%	85.00%	95.70%
Return of capital	11.60%	4.60%	3.00%
Capital gains	2.50%	10.40%	1.30%
Total	100.00%	100.00%	100.00%
Dividends declared	$ 2.61	$ 2.09	$ 1.86
Series C Preferred Stock
Income Tax Treatment of Distributions and Dividends [Line Items]
Ordinary income		89.10%	98.60%
Return of capital		0.00%	0.00%
Capital gains		10.90%	1.40%
Total		100.00%	100.00%
Dividends declared		$ 0.5234375	$ 2.09375
Series D Convertible Preferred Stock
Income Tax Treatment of Distributions and Dividends [Line Items]
Ordinary income	97.20%	89.10%	98.60%
Return of capital	0.00%	0.00%	0.00%
Capital gains	2.80%	10.90%	1.40%
Total	100.00%	100.00%	100.00%
Dividends declared	$ 1.75	$ 1.75	$ 1.75
Series E Cumulative Redeemable Preferred Stock
Income Tax Treatment of Distributions and Dividends [Line Items]
Ordinary income	97.20%	89.10%
Return of capital	0.00%	0.00%
Capital gains	2.80%	10.90%
Total	100.00%	100.00%
Dividends declared	$ 1.61250	$ 1.343750