Fair Values of Certain Financial Liabilities and Other Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Quoted Prices in Active Markets for Identical Assets (Level 1) | Carrying (Reported) Amount, Fair Value Disclosure | Senior Notes
Financial liabilities
Senior notes (Level 1)	$ 2,647	$ 3,038
Quoted Prices in Active Markets for Identical Assets (Level 1) | Carrying (Reported) Amount, Fair Value Disclosure | Exchangeable Senior Debentures
Financial liabilities
Exchangeable Senior Debentures (Level 1)	367	531
Quoted Prices in Active Markets for Identical Assets (Level 1) | Portion at Fair Value, Fair Value Disclosure | Senior Notes
Financial liabilities
Senior notes (Level 1)	2,758	3,296
Quoted Prices in Active Markets for Identical Assets (Level 1) | Portion at Fair Value, Fair Value Disclosure | Exchangeable Senior Debentures
Financial liabilities
Exchangeable Senior Debentures (Level 1)	562	725
Significant Other Observable Inputs (Level 2) | Carrying (Reported) Amount, Fair Value Disclosure | Credit Facility
Financial liabilities
Credit facility (Level 2)	729	763
Significant Other Observable Inputs (Level 2) | Carrying (Reported) Amount, Fair Value Disclosure | Mortgage Debt and Other, Net of Capital Leases
Financial liabilities
Mortgage debt and other, net of capital leases (Level 2)	816	1,078
Significant Other Observable Inputs (Level 2) | Portion at Fair Value, Fair Value Disclosure | Credit Facility
Financial liabilities
Credit facility (Level 2)	729	763
Significant Other Observable Inputs (Level 2) | Portion at Fair Value, Fair Value Disclosure | Mortgage Debt and Other, Net of Capital Leases
Financial liabilities
Mortgage debt and other, net of capital leases (Level 2)	$ 831	$ 1,094