Derivative Instruments - Narrative (Detail) (USD $)	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Interest rate swaps - weighted average pay rate, percentage	2.55%	3.21%
Interest rate swaps - weighted average receive rate, percentage	0.33%	0.28%
Interest rate swaps, gross unrealized loss	$ 2,600,000,000	$ 820,000,000
Interest rate swaps, notional amount related to gross unrealized loss	40,100,000,000	23,200,000,000
Interest rate swaps, gross unrealized gain		68,200,000
Interest rate swaps, notional amount related to gross unrealized gain		$ 3,900,000,000