Income taxes Income Tax Treatment of Distributions (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Common Stock
Income Tax Treatment of Distributions and Dividends [Line Items]
Ordinary income	91.80%	85.90%	85.00%
Return of capital	8.20%	11.60%	4.60%
Capital gains	0.00%	2.50%	10.40%
Total	100.00%	100.00%	100.00%
Dividends declared	$ 2.88	$ 2.61	$ 2.09
Series C Preferred Stock
Income Tax Treatment of Distributions and Dividends [Line Items]
Ordinary income			89.10%
Return of capital			0.00%
Capital gains			10.90%
Total			100.00%
Dividends declared			$ 0.5234375
Series D Convertible Preferred Stock
Income Tax Treatment of Distributions and Dividends [Line Items]
Ordinary income	100.00%	97.20%	89.10%
Return of capital	0.00%	0.00%	0.00%
Capital gains	0.00%	2.80%	10.90%
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	100.00%	97.20%	89.10%
Return of capital	0.00%	0.00%	0.00%
Capital gains	0.00%	2.80%	10.90%
Total	100.00%	100.00%	100.00%
Dividends declared	$ 1.6125	$ 1.6125	$ 1.34375

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Represents the percentage of the distributions and dividends declared during the period treated as capital gains at 15 percent for income tax purposes.

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Represents the percentage of the distributions and dividends declared during the period treated as ordinary income for income tax purposes.

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Represents the percentage summation of distributions and dividends declared during the period for an income tax treatment disclosure.

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- Definition

Represents the percentage of the distributions and dividends declared during the period treated as a return of capital for income tax purposes.

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- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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