Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011 Interest Rates [Member]	Dec. 31, 2010 Interest Rates [Member]	Dec. 31, 2011 Equity Contract	Dec. 31, 2010 Equity Contract	Dec. 31, 2009 Derivative
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative assets	$ 34	$ 13	$ 113	$ 81	$ 84
Derivative liabilities	$ 2	$ 11	$ 81	$ 28	$ 75