Interest Expense for Debentures (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Debt Instrument [Line Items]
Total interest expense	$ 373	[1]	$ 371	[1]	$ 383	[1]
Exchangeable Senior Debentures
Debt Instrument [Line Items]
Contractual interest expense (cash)	19		31		34
Non-cash interest expense due to discount amortization	17		31		32
Total interest expense	$ 36		$ 62		$ 66

[1] Interest expense and interest paid for 2012, 2011 and 2010 includes cash prepayment premiums of approximately $21 million, $5 million and $20 million, respectively.