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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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Debt instrument maturity year.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Fair value amount of debt instrument that was repurchased.

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Amount of the cost of borrowed funds accounted for as interest expense.

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Amount of cash paid for interest. Includes, but is not limited to, payment to settle zero-coupon bond attributable to accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount.

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The amount of expense provided in the period for legal costs incurred on or before the balance sheet date pertaining to resolved, pending or threatened litigation, including arbitration and mediation proceedings.

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Interest received on loans and other debt instruments during the current period.

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