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Derivative Instruments (Narrative) (Detail) $ in Millions | Sep. 30, 2020 USD ($) | Dec. 31, 2019 USD ($)
--- | --- | ---
Derivative [Line Items] |  |  |
Cash collateral posted to derivative counterparties | $ 0 | $ 0 |
Securities posted as collateral to derivative counterparties | $ 237 | $ 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 | 2,566 | 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 | 1,785 | 1,957 |
Insurance Operations [Member] | Insured Swaps [Member] |  |  |
Derivative [Line Items] |  |  |
Insured swaps | 1,762 | 1,925 | [1]
Insurance Operations [Member] | Credit Rating Aa [Member] |  |  |
Derivative [Line Items] |  |  |
Insured swaps | 59 | 121 |
Insurance Operations [Member] | Credit Rating Aa [Member] | Insured Swaps [Member] |  |  |
Derivative [Line Items] |  |  |
Insured swaps | 59 | 121 | [2]
Insurance Operations [Member] | Credit Rating A [Member] |  |  |
Derivative [Line Items] |  |  |
Insured swaps | 1,341 | 1,371 |
Insurance Operations [Member] | Credit Rating A [Member] | Insured Swaps [Member] |  |  |
Derivative [Line Items] |  |  |
Insured swaps | 1,341 | 1,371 | [3]
Insurance Operations [Member] | Credit Rating Bbb [Member] |  |  |
Derivative [Line Items] |  |  |
Insured swaps | 362 | 433 |
Insurance Operations [Member] | Credit Rating Bbb [Member] | Insured Swaps [Member] |  |  |
Derivative [Line Items] |  |  |
Insured swaps | $ 362 | 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.

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