Derivative Instruments (Total Fair Value Of Company's Derivative Assets And Liabilities By Instrument And Balance Sheet Location, Before Counterparty Netting) (Detail) - Derivative Instrument [Member] - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivative [Line Items]
Derivative notional amount	$ 2,382	$ 2,534
Derivative Assets, Not designated, Fair Value	10	7
Derivative Liabilities, Not designated, Fair Value	(141)	(225)
Insured swaps [Member]
Derivative [Line Items]
Derivative notional amount	1,696	1,743
Insured swaps [Member] | Other assets [Member]
Derivative [Line Items]
Derivative Assets, Not designated, Fair Value	0	0
Insured swaps [Member] | Derivative liabilities [Member]
Derivative [Line Items]
Derivative Liabilities, Not designated, Fair Value	(1)	(2)	[1]
Interest rate swaps [Member]
Derivative [Line Items]
Derivative notional amount	425	437
Interest rate swaps [Member] | Other assets [Member]
Derivative [Line Items]
Derivative Assets, Not designated, Fair Value	1	1
Interest rate swaps [Member] | Derivative liabilities [Member]
Derivative [Line Items]
Derivative Liabilities, Not designated, Fair Value	(132)	(164)
Interest rate swaps-embedded [Member]
Derivative [Line Items]
Derivative notional amount	211	252
Interest rate swaps-embedded [Member] | Medium-term notes [Member]
Derivative [Line Items]
Derivative Assets, Not designated, Fair Value	0	0
Derivative Liabilities, Not designated, Fair Value	(8)	(10)
Currency swaps-VIE [Member]
Derivative [Line Items]
Derivative notional amount	50	53
Currency swaps-VIE [Member] | Other assets-VIE [Member]
Derivative [Line Items]
Derivative Assets, Not designated, Fair Value	9	6
Currency swaps-VIE [Member] | Derivative liabilities-VIE [Member]
Derivative [Line Items]
Derivative Liabilities, Not designated, Fair Value	$ 0	0
All other [Member]
Derivative [Line Items]
Derivative notional amount		49
All other [Member] | Other assets [Member]
Derivative [Line Items]
Derivative Assets, Not designated, Fair Value		0
All other [Member] | Derivative liabilities [Member]
Derivative [Line Items]
Derivative Liabilities, Not designated, Fair Value		$ (49)

[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.