Business Segments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2012 segment	Dec. 31, 2011
Operating Statistics [Line Items]
Number of Reportable Segments		1
Actuarial losses of projected benefit obligation			10.00%
Recognized in earnings through amendment to benefit formula			$ 31
Curtailment gain	52
Interest Rate Contracts [Member]
Operating Statistics [Line Items]
IFRS Cumulative adjustment to correct prior period errors			$ 185