Variable Interest Entities (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013 Entity	Dec. 31, 2012	Mar. 31, 2012 Entity
Variable Interest Entity [Line Items]
Total VIE assets	$ 135.0
Carrying amounts of assets	21,592	21,724	26,126
Carrying amounts of liabilities	18,283	18,530
Additional VIEs were consolidated during	0		0
Variable Interest Entity [Member]
Variable Interest Entity [Line Items]
Carrying amounts of assets	8,300	8,300
Carrying amounts of liabilities	$ 7,200	$ 7,300