Debt (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Senior notes	$ 2,376	$ 2,858
Exchangeable senior debentures		386
Credit facility	1,291	697
Mortgage debt (non-recourse), with an average interest rate of 4.7% and 5.0% at December 31, 2015 and 2014, respectively, maturing through January 2024	350	402
Total debt	4,017	3,957
Series V senior notes 6% due November 2020
Debt Instrument [Line Items]
Senior notes		495
Series X senior notes 5.875% due June 2019
Debt Instrument [Line Items]
Senior notes		493
Series Z senior notes 6% due October 2021
Debt Instrument [Line Items]
Senior notes	297	296
Series B senior notes 5.25% due March 2022
Debt Instrument [Line Items]
Senior notes	347	346
Series C senior notes 4.75% due March 2023
Debt Instrument [Line Items]
Senior notes	445	445
Series D senior notes 3.75% due October 2023
Debt Instrument [Line Items]
Senior notes	397	397
Series E senior notes 4% due June 2025
Debt Instrument [Line Items]
Senior notes	495
Series F senior notes 4.5% due February 2026
Debt Instrument [Line Items]
Senior notes	395
Exchangeable 2009 Senior Debentures 2.5% Due October 2029
Debt Instrument [Line Items]
Exchangeable senior debentures		386
Term Loan due June 2017
Debt Instrument [Line Items]
Credit facility	499	499
Term Loan due September 2020
Debt Instrument [Line Items]
Credit facility	497
Revolver
Debt Instrument [Line Items]
Credit facility	$ 295	$ 198

Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
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Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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