Derivative Instruments (Total Fair Value Of Company's Derivative Assets And Liabilities By Instrument And Balance Sheet Location, Before Counterparty Netting) (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative notional amount		$ 3,196	$ 3,619
Derivative Liabilities [Member]
Derivative [Line Items]
Derivative Liabilities, Not designated, Fair Value	[1]	(213)	(212)
Derivative Assets [Member]
Derivative [Line Items]
Derivative Assets, Not designated, Fair Value	[1]	15	18
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative notional amount		683	712
Interest Rate Swap [Member] | Derivative Liabilities [Member]
Derivative [Line Items]
Derivative Liabilities, Not designated, Fair Value	[1]	(172)	(157)
Interest Rate Swap [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative Assets, Not designated, Fair Value	[1]	1	2
Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount		53	70
Credit Default Swap [Member] | Derivative Liabilities [Member]
Derivative [Line Items]
Derivative Liabilities, Not designated, Fair Value	[1]	(19)	(33)
Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount		2,063	2,433
Insured Swaps [Member] | Derivative Liabilities [Member]
Derivative [Line Items]
Derivative Liabilities, Not designated, Fair Value	[1]	(2)	(2)
Interest Rate Swaps V I E [Member]
Derivative [Line Items]
Derivative notional amount		0	0
Interest Rate Swaps V I E [Member] | Derivative Liabilities V I E [Member]
Derivative [Line Items]
Derivative Liabilities, Not designated, Fair Value	[1]	0	0
Interest Rate Swaps V I E [Member] | Derivative Assets V I E [Member]
Derivative [Line Items]
Derivative Assets, Not designated, Fair Value		0
Interest Rate Swaps Embedded [Member]
Derivative [Line Items]
Derivative notional amount		287	293
Interest Rate Swaps Embedded [Member] | Medium Term Notes [Member]
Derivative [Line Items]
Derivative Assets, Not designated, Fair Value	[1]	0	0
Derivative Liabilities, Not designated, Fair Value	[1]	(13)	(13)
Currency Swaps Vie [Member]
Derivative [Line Items]
Derivative notional amount		61	62
Currency Swaps Vie [Member] | Other Assets V I E [Member]
Derivative [Line Items]
Derivative Assets, Not designated, Fair Value	[1]	14	16
All Other [Member]
Derivative [Line Items]
Derivative notional amount		49	49
All Other [Member] | Derivative Liabilities [Member]
Derivative [Line Items]
Derivative Liabilities, Not designated, Fair Value	[1]	(7)	(7)
All Other V I E [Member]
Derivative [Line Items]
Derivative notional amount		0	0
All Other V I E [Member] | Other Assets V I E [Member]
Derivative [Line Items]
Derivative Assets, Not designated, Fair Value		0
All Other Embedded [Member]
Derivative [Line Items]
Derivative notional amount		0	0
All Other Embedded [Member] | Derivative Liabilities Other Investments [Member]
Derivative [Line Items]
Derivative Liabilities, Not designated, Fair Value	[1]	0	0
Insured Swaps - Held for Sale [Member]
Derivative [Line Items]
Derivative notional amount		$ 0	$ 0

[1] In accordance with the accounting guidance for derivative instruments and hedging activities, the balance sheet location of the Company's embedded derivative instruments is determined by the location of the related host contract