Equity (Preferred Stock) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Preferred Stock [Member]
Preferred Stock
Declaration Date	Nov. 17, 2014	Aug. 15, 2014	May 15, 2014	Mar. 05, 2014	Nov. 15, 2013	Aug. 15, 2013	May 15, 2013	Mar. 05, 2013	Nov. 15, 2012	Aug. 15, 2012	May 15, 2012	Mar. 05, 2012
Record Date	Nov. 30, 2014	Aug. 31, 2014	May 31, 2014	Feb. 28, 2014	Nov. 30, 2013	Aug. 31, 2013	May 31, 2013	Feb. 28, 2013	Nov. 30, 2012	Aug. 31, 2012	May 31, 2012	Feb. 29, 2012
Series A Preferred Stock [Member]
Preferred Stock
Preferred stock, dividend rate	$ 0.253	$ 0.256	$ 0.256	$ 0.250	$ 0.253	$ 0.256	$ 0.256	$ 0.250	$ 0.253	$ 0.256	$ 0.256	$ 0.253
Preferred stock, dividends	$ 7	$ 6	$ 7	$ 6	$ 7	$ 6	$ 7	$ 6	$ 7	$ 6	$ 7	$ 6	$ 26	$ 26	$ 26
Series B Preferred Stock [Member]
Preferred Stock
Preferred stock, dividend rate	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406	$ 0.406
Preferred stock, dividends	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 24	$ 96	$ 96	$ 96

Date the dividend to be paid was declared, in CCYY-MM-DD format.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586

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Date the holder must own the stock to be entitled to the dividend, in CCYY-MM-DD format.

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Amount of paid and unpaid preferred stock dividends declared with the form of settlement in cash.

+ 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

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 405

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6957935&loc=d3e64057-112817

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The amount per share used to calculated dividend payments on preferred stock.

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