Derivative Instruments (Narrative) (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Maximum amount of future guarantee payments	$ 792
Cash collateral posted to derivative counterparties	37	$ 50
Securities posted as collateral to derivative counterparties	341	259
Fair value of Credit Support Annex	3	3
Amount of counterparty netting (assets)	1	1
Derivative assets, after counterparty netting	20	19
Other assets and other assets of consolidated VIEs, after counterparty netting and embedded reclass	16	14
Amount of counterparty netting (liabilities)	1	1
Derivative liabilities, after counterparty netting and posting of cash collateral	403	374
Derivative liabilities and derivative liabilities of consolidated VIEs, after counterparty netting and embedded reclass	383	359
Derivative Liabilities [Member]
Derivative [Line Items]
Cash collateral posted to derivative counterparties	18	31
Other Liabilities [Member]
Derivative [Line Items]
Cash collateral posted to derivative counterparties	19	19
Other Assets [Member]
Derivative [Line Items]
Cash collateral posted to derivative counterparties		0
Medium Term Notes And Other Investments [Member] | Embedded [Member]
Derivative [Line Items]
Embedded derivative assets	4	5
Embedded derivative liabilities	$ 20	$ 15