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Debt - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions |  | 1 Months Ended | 3 Months Ended | 9 Months Ended
--- | --- | --- | --- | ---
Jun. 19, 2017 | May 31, 2017 | Apr. 30, 2017 | Feb. 28, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Repayment of debt |  |  |  |  |  | $ 409 |  |  |
Common stock sold |  |  |  | 23,353,000 |  | 12,958,767 |  |  |
Common stock sold per share |  |  |  | $ 38.03 |  | $ 35.91 |  |  |
Pre-tax gain on sale of common stock |  |  |  | $ 351 |  |  |  | $ 167 |
Consideration amount including accrued and unpaid interest |  |  |  |  |  | $ 874 |  |  |
Interest expense |  |  |  |  |  |  |  | 58 |
Interest paid |  |  |  |  |  |  |  | 27 |
Legal fees |  |  |  |  |  |  |  | 2 |
Interest expense for premium paid for early redemption of notes in excess of carrying value | $ 24 |  |  |  | $ 100 |  | $ 126 | $ 398 | $ 371
6.50% Bonds, Due 2018 [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, interest rate |  |  |  |  | 6.50% |  |  | 6.50% |
Debt instrument, maturity date |  |  |  |  |  |  |  | 2018 |
6.50% Senior Notes Due 2018 [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt | 100 |  |  |  |  |  |  |  |
6.75% Notes, Due 2018 [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, interest rate |  |  |  |  | 6.75% |  |  | 6.75% |
Debt instrument, maturity date |  |  |  |  |  |  |  | 2018 |
6.75% Senior Notes Due 2018 [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt | 345 |  |  |  |  |  |  |  |
Redemption amount | $ 479 |  |  |  |  |  |  |  |
5.72% Notes, Due 2019 [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, interest rate |  |  |  |  | 5.72% |  |  | 5.72% |
Debt instrument, maturity date |  |  |  |  |  |  |  | 2019 |
Investment Banks [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  | $ 805 |  |  | $ 805 |
Common stock sold |  | 12,958,767 | 12,958,767 |  |  |  |  |  |
Common stock sold per share |  | $ 35.91 | $ 35.91 |  |  |  |  |  |
Pre-tax gain on sale of common stock |  | $ 167 | $ 167 |  |  |  |  |  |
Investment Banks [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt |  |  | $ 1,000 |  |  |  |  |  |
Investment Banks [Member] | 6.50% Bonds, Due 2018 [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, interest rate |  |  | 6.50% |  | 6.50% |  |  | 6.50% |
Debt instrument, maturity date |  |  | 2018 |  |  |  |  |  |
Long-term debt |  |  |  |  | $ 150 |  |  | $ 150 |
Investment Banks [Member] | 6.75% Notes, Due 2018 [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, interest rate |  |  | 6.75% |  | 6.75% |  |  | 6.75% |
Debt instrument, maturity date |  |  | 2018 |  |  |  |  |  |
Long-term debt |  |  |  |  | $ 405 |  |  | $ 405 |
Investment Banks [Member] | 5.72% Notes, Due 2019 [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, interest rate |  |  | 5.72% |  | 5.72% |  |  | 5.72% |
Debt instrument, maturity date |  |  | 2019 |  |  |  |  |  |
Long-term debt |  |  |  |  | $ 250 |  |  | $ 250 |
Interest Rate Swap [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Proceeds from interest rate swap agreement |  |  |  |  |  |  |  | $ 8 |

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