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Debt (Public Debt) (Detail) |  |  | 1 Months Ended | 2 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Oct. 02, 2017 shares | Jun. 19, 2017 USD ($) | Dec. 31, 2016 USD ($) | Sep. 30, 2014 shares | May 31, 2017 USD ($) $ / shares shares | Jun. 30, 2017 USD ($) $ / shares shares | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Apr. 30, 2017 USD ($) cash_tender_offer
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Cash paid to holders of debt instrument, net |  |  |  |  |  | $ 409,000,000 |  |  |  |
Common stock sold (in shares) | shares | 24,022 |  |  | 25,000,000 |  | 12,958,767 |  |  |  |
Common stock sold per share (usd per share) | $ / shares |  |  |  |  |  | $ 35.91 |  |  |  |
Debt instrument, repurchase amount |  |  |  |  |  | $ 874,000,000 |  |  |  |
Interest expense, debt |  |  |  |  |  | 58,000,000 |  |  |  |
Interest paid |  |  |  |  |  | 27,000,000 |  |  |  |
Legal fees |  |  |  |  |  | 2,000,000 |  |  |  |
Pre-tax gain on sale of common stock |  |  |  |  |  | 167,000,000 | $ 167,000,000 |  |  |
Interest expense (S) |  | $ 24,000,000 |  |  |  |  | 496,000,000 | $ 499,000,000 | $ 473,000,000 |
Principal amount of convertible notes |  |  | $ 8,048,000,000 |  |  |  | 6,807,000,000 | $ 8,048,000,000 |  |
6.50% Bonds, due 2018 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Convertible notes, interest rate |  |  | 6.50% |  |  |  |  | 6.50% |  |
Principal amount of convertible notes |  |  | $ 250,000,000 |  |  |  | 0 | $ 250,000,000 |  |
6.75% Notes, due 2018 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Convertible notes, interest rate |  |  | 6.75% |  |  |  |  | 6.75% |  |
Principal amount of convertible notes |  |  | $ 750,000,000 |  |  |  | $ 0 | $ 750,000,000 |  |
5.72% Notes, due 2019 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Convertible notes, interest rate |  |  |  |  |  |  | 5.72% |  |  |
Principal amount of convertible notes |  |  | $ 750,000,000 |  |  |  | $ 500,000,000 | $ 750,000,000 |  |
6.5% Senior Notes Due 2018 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  | 100,000,000 |  |  |  |  |  |  |  |
6.75% Senior Notes Due 2018 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  | 345,000,000 |  |  |  |  |  |  |  |
Debt instrument redemption amount |  | $ 479,000,000 |  |  |  |  |  |  |  |
5.55% Notes Due February 2017 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Convertible notes, interest rate |  |  | 5.55% |  |  |  |  | 5.55% |  |
Cash paid to holders of debt instrument, net |  |  | $ 770,000,000 |  |  |  |  |  |  |
Principal amount of convertible notes |  |  | 750,000,000 |  |  |  |  | $ 750,000,000 |  |
Cash paid to holders of debt instrument, portion of interest |  |  | 17,000,000 |  |  |  |  |  |  |
Cash paid to holders of debt instrument, portion of purchase premium |  |  | $ 3,000,000 |  |  |  |  |  |  |
Investment Banks |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Number of cash tender offers | cash_tender_offer |  |  |  |  |  |  |  |  |  | 3
Debt instrument, face amount |  |  |  |  |  |  |  |  |  | $ 805,000,000
Common stock sold (in shares) | shares |  |  |  |  | 12,958,767 |  |  |  |  |
Common stock sold per share (usd per share) | $ / shares |  |  |  |  | $ 35.91 |  |  |  |  |
Pre-tax gain on sale of common stock |  |  |  |  | $ 167,000,000 |  |  |  |  |
Investment Banks | 6.50% Bonds, due 2018 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  |  |  |  | 150,000,000
Investment Banks | 6.75% Notes, due 2018 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  |  |  |  | 405,000,000
Investment Banks | 5.72% Notes, due 2019 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  |  |  |  | 250,000,000
Interest Rate Swap |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Proceeds from interest received |  |  |  |  |  | $ 8,000,000 |  |  |  |
Maximum | Investment Banks |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  |  |  |  | $ 1,000,000,000

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