Fair Values of Certain Financial Liabilities and Other Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Carrying (Reported) Amount, Fair Value Disclosure | Quoted Prices in Active Markets for Identical Assets (Level 1) | Senior Notes
Financial liabilities
Financial liabilities	$ 3,439	$ 3,038
Carrying (Reported) Amount, Fair Value Disclosure | Quoted Prices in Active Markets for Identical Assets (Level 1) | Exchangeable Senior Debentures
Financial liabilities
Financial liabilities	359	531
Carrying (Reported) Amount, Fair Value Disclosure | Significant Other Observable Inputs (Level 2) | Credit Facility
Financial liabilities
Financial liabilities	658	763
Carrying (Reported) Amount, Fair Value Disclosure | Significant Other Observable Inputs (Level 2) | Mortgage debt and other, net of capital leases
Financial liabilities
Financial liabilities	1,077	1,078
Portion at Fair Value, Fair Value Disclosure | Quoted Prices in Active Markets for Identical Assets (Level 1) | Senior Notes
Financial liabilities
Financial liabilities	3,715	3,296
Portion at Fair Value, Fair Value Disclosure | Quoted Prices in Active Markets for Identical Assets (Level 1) | Exchangeable Senior Debentures
Financial liabilities
Financial liabilities	558	725
Portion at Fair Value, Fair Value Disclosure | Significant Other Observable Inputs (Level 2) | Credit Facility
Financial liabilities
Financial liabilities	658	763
Portion at Fair Value, Fair Value Disclosure | Significant Other Observable Inputs (Level 2) | Mortgage debt and other, net of capital leases
Financial liabilities
Financial liabilities	$ 1,095	$ 1,094