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Capital Structure - Changes in Redeemable Preferred Stock (Details) - USD ($) $ in Millions |  |  | 12 Months Ended
--- | --- | --- | ---
Jun. 13, 2016 | May 24, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Increase (Decrease) in Temporary Equity [Roll Forward] |  |  |  |  |
Balance at beginning of period |  |  | $ 0 | $ 302 | $ 291
Accretion to redemption value |  |  |  | 2 | 11
Repurchase of 2.822% redeemable preferred stock |  |  | 0 | (226) | 0
Gain on redemption of 2.822% redeemable preferred stock |  |  | 0 | 78 | 0
Balance at end of period |  |  | $ 0 | $ 0 | $ 302
Convertible Preferred Stock |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Preferred stock, dividend rate (as percent) |  | 2.822% | 2.822% | 2.822% | 2.822%
Increase (Decrease) in Temporary Equity [Roll Forward] |  |  |  |  |
Repurchase of 2.822% redeemable preferred stock | $ (226) |  |  |  |
Gain on redemption of 2.822% redeemable preferred stock | 78 |  |  |  |
Balance at end of period | $ 304 |  |  |  |

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- Definition The cash outflow for reacquisition of callable preferred stock. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585 + Details
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- Definition Carrying amount, attributable to parent, of an entity's issued and outstanding stock which is not included within permanent equity. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. Includes stock with a put option held by an ESOP and stock redeemable by a holder only in the event of a change in control of the issuer. + References No definition available. + Details
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