Stockholders' Equity (Preferred Stock Issued and Outstanding) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Shares issued (in shares)	1,761,000	2,241,000
Shares outstanding (in shares)	1,761,000	2,241,000
Carrying Value	$ 2,793	$ 2,783
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Shares issued (in shares)	0	485,000
Shares outstanding (in shares)	0	485,000
Liquidation preference (USD per share)	$ 1,000
Carrying Value	$ 0	$ 482
Dividend Rate in Effect	0.00%
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Shares issued (in shares)	600,000	600,000
Shares outstanding (in shares)	600,000	600,000
Liquidation preference (USD per share)	$ 1,000
Carrying Value	$ 585	$ 585
Dividend Rate in Effect	6.00%
Series D Preferred Stock [Member]
Class of Stock [Line Items]
Shares issued (in shares)	750,000	750,000
Shares outstanding (in shares)	750,000	750,000
Liquidation preference (USD per share)	$ 1,000
Carrying Value	$ 728	$ 728
Dividend Rate in Effect	5.95%
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Shares issued (in shares)	400,000	400,000
Shares outstanding (in shares)	400,000	400,000
Liquidation preference (USD per share)	$ 1,000
Carrying Value	$ 397	$ 397
Dividend Rate in Effect	7.00%
Series A Preferred Stock [Member] \| LIBOR [Member]
Class of Stock [Line Items]
Floating Annual Rate	4.82%
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Shares issued (in shares)	6,000	6,000
Shares outstanding (in shares)	6,000	6,000
Liquidation preference (USD per share)	$ 100,000
Carrying Value	$ 591	$ 591
Dividend Rate in Effect	4.625%
Series E Preferred Stock [Member] \| LIBOR [Member]
Class of Stock [Line Items]
Floating Annual Rate	3.315%
Series F Preferred Stock [Member]
Class of Stock [Line Items]
Shares issued (in shares)	5,000	0
Shares outstanding (in shares)	5,000	0
Liquidation preference (USD per share)	$ 100,000
Carrying Value	$ 492	$ 0
Dividend Rate in Effect	5.00%
Series F Preferred Stock [Member] \| LIBOR [Member]
Class of Stock [Line Items]
Floating Annual Rate	2.575%

The percentage points added to the reference rate to compute the variable rate.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The percentage rate used to calculate dividend payments on preferred stock.

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The per share liquidation preference (or restrictions) of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

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Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
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Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased.

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Aggregate par or stated value of issued nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer). This item includes treasury stock repurchased by the entity. Note: elements for number of nonredeemable preferred shares, par value and other disclosure concepts are in another section within stockholders' equity.

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