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Fair Value Of Financial Instruments (Fair Value Hierarchy Table Presents The Company's Assets And Liabilities At Fair Value Not Recorded On The Company's Consolidated Balance Sheet) (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Variable Interest Entity Primary Beneficiary [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes	$ 451	$ 623
Carrying Reported Amount Fair Value Disclosure [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Long-term debt	2,258	2,229
Medium-term notes	590	598
Investment agreements	268	269
Total liabilities	3,242	3,369
Gross	(246)	(282)
Ceded recoverable (liability)	(19)	(17)
Carrying Reported Amount Fair Value Disclosure [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes	126	273
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Long-term debt	736	631
Medium-term notes	395	396
Investment agreements	353	376
Total liabilities	1,613	1,679
Gross	874	811
Ceded recoverable (liability)	45	45
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes	129	276
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 1 [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Long-term debt	0	0
Medium-term notes	0	0
Investment agreements	0	0
Total liabilities	0	0
Gross	0	0
Ceded recoverable (liability)	0	0
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 1 [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes	0	0
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 2 [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Long-term debt	736	631
Medium-term notes	0	0
Investment agreements	0	0
Total liabilities	865	907
Gross	0	0
Ceded recoverable (liability)	0	0
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 2 [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes	129	276
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 3 [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Long-term debt	0	0
Medium-term notes	395	396
Investment agreements	353	376
Total liabilities	748	772
Gross	874	811
Ceded recoverable (liability)	45	45
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 3 [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes	$ 0	$ 0

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- Definition

Represents the Company's balance sheet carrying amount of the financial guarantee insurance contracts ceded to third party reinsurers. The balance sheet carrying amount comprises the prepaid reinsurance premiums and reinsurance recoverable on paid and unpaid losses

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Represents the Company's balance sheet carrying amount of the financial guarantee insurance contract obligations with third parties. The gross balance sheet carrying amount comprises the unearned premium revenue and the loss and loss adjustment expense reserves balances

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Fair value of financial and nonfinancial obligations.

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Including the current and noncurrent portions, carrying value at the balance sheet date of a financial instrument used to receive debt funding on a regular basis with original maturities ranging from 5-10 years.

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