Derivative Instruments (Narrative) (Detail) $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Derivative [Line Items]
Cash collateral posted to derivative counterparties	$ 0	$ 0
Securities posted as collateral to derivative counterparties	$ 214	$ 181
Number of credit support annexes	1	1
Fair value of Credit Support Annex	$ 1	$ 1
Derivative Instrument [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Insured swaps	2,534	2,737
Insured Swaps [Member] | Derivative Instrument [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Insured swaps		1,925
Insurance Operations [Member]
Derivative [Line Items]
Insured swaps	1,743	1,957
Insurance Operations [Member] | Insured Swaps [Member]
Derivative [Line Items]
Insured swaps	1,743	1,925	[1]
Insurance Operations [Member] | Credit Rating Aa [Member]
Derivative [Line Items]
Insured swaps	58	121
Insurance Operations [Member] | Credit Rating Aa [Member] | Insured Swaps [Member]
Derivative [Line Items]
Insured swaps	58	121	[2]
Insurance Operations [Member] | Credit Rating A [Member]
Derivative [Line Items]
Insured swaps	1,327	1,371
Insurance Operations [Member] | Credit Rating A [Member] | Insured Swaps [Member]
Derivative [Line Items]
Insured swaps	1,327	1,371	[3]
Insurance Operations [Member] | Credit Rating Bbb [Member]
Derivative [Line Items]
Insured swaps	358	433
Insurance Operations [Member] | Credit Rating Bbb [Member] | Insured Swaps [Member]
Derivative [Line Items]
Insured swaps	$ 358	433	[4]
Previously Reported [Member] | Insured Swaps [Member] | Derivative Instrument [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Insured swaps		1,795
Previously Reported [Member] | Insurance Operations [Member] | Insured Swaps [Member]
Derivative [Line Items]
Insured swaps		1,795
Previously Reported [Member] | Insurance Operations [Member] | Credit Rating Aa [Member] | Insured Swaps [Member]
Derivative [Line Items]
Insured swaps		66
Previously Reported [Member] | Insurance Operations [Member] | Credit Rating A [Member] | Insured Swaps [Member]
Derivative [Line Items]
Insured swaps		1,284
Previously Reported [Member] | Insurance Operations [Member] | Credit Rating Bbb [Member] | Insured Swaps [Member]
Derivative [Line Items]
Insured swaps		$ 445

[1] The Company revised its previously reported amount of $1,795 million to $1,925 million.

[2] The Company revised its previously reported amount of $66 million to $121 million.

[3] The Company revised its previously reported amount of $1,284 million to $1,371 million.

[4] The Company revised its previously reported amount of $445 million to $433 million.