Fair Value Of Financial Instruments (Company's Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Sep. 30, 2019		Dec. 31, 2018
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	$ 4,315		$ 5,131
Fair value financial liabilities measured on recurring basis	776		786
Counterparty and cash collateral netting	0
Money Market Securities [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	164		67	[1]
Medium-term Notes [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	106		102
Perpetual Debt And Equity Securities [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	65		58
Cash and Cash Equivalents [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	76		222
Fixed Income Funds [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	52	[1]	75
Fixed Maturities [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	3,214		3,871
Fixed Maturities [Member] | U S Treasury And Government [Member] | Other Fixed Maturity Investments [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	874		1,118
Fixed Maturities [Member] | State and municipal bonds [Member] | Other Fixed Maturity Investments [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	230		728
Fixed Maturities [Member] | Foreign Government Debt [Member] | Other Fixed Maturity Investments [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	9		9
Fixed Maturities [Member] | Corporate Obligations [Member] | Other Fixed Maturity Investments [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	1,333		1,410
Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member] | Mortgage-backed [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	310		219
Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	21		28
Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	26		54
Fixed Maturities [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	130		121
Fixed Maturities [Member] | Other Asset Backed [Member] | Asset-backed [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	281		184
Derivative Assets [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	2		2
Assets Of Consolidated V I Es [Member] | Variable Interest Entity, Primary Beneficiary [Member] | Loan Repurchase Commitments [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	478		418
Assets Of Consolidated V I Es [Member] | Corporate Obligations [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	8		14
Assets Of Consolidated V I Es [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	47		92
Assets Of Consolidated V I Es [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	16		34
Assets Of Consolidated V I Es [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	6		7
Assets Of Consolidated V I Es [Member] | Other Asset Backed [Member] | Asset-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	11		10
Assets Of Consolidated V I Es [Member] | Loans Receivable and Other Instruments At Fair Value [Member] | Variable Interest Entity, Primary Beneficiary [Member] | Residential Loans Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	146		172
Assets Of Consolidated V I Es [Member] | Currency Derivatives [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	8		17
Assets Of Consolidated V I Es [Member] | Cash and Cash Equivalents [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	5		58
Assets Of Consolidated V I Es [Member] | Other [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	17		14
Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	12		35
Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	152		157
Derivative Liabilities [Member] | Other Derivatives [Member] | Non-Insured Derivatives [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	30		7
Other Liabilities [Member] | Other Payable [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	4		5
Liabilities Of Consolidated Vies [Member] | Variable Interest Entity Notes [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	472		480
Fair Value Inputs Level 1 [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	1,048		1,331
Fair Value Inputs Level 1 [Member] | Money Market Securities [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	164
Fair Value Inputs Level 1 [Member] | Perpetual Debt And Equity Securities [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	28		23
Fair Value Inputs Level 1 [Member] | Cash and Cash Equivalents [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	76		222
Fair Value Inputs Level 1 [Member] | Fixed Maturities [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	775		1,028
Fair Value Inputs Level 1 [Member] | Fixed Maturities [Member] | U S Treasury And Government [Member] | Other Fixed Maturity Investments [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	775		1,028
Fair Value Inputs Level 1 [Member] | Assets Of Consolidated V I Es [Member] | Other Asset Backed [Member] | Asset-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	0
Fair Value Inputs Level 1 [Member] | Assets Of Consolidated V I Es [Member] | Cash and Cash Equivalents [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	5		58
Fair Value Inputs Level 2 [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	2,565		3,021
Fair value financial liabilities measured on recurring basis	278		273
Fair Value Inputs Level 2 [Member] | Perpetual Debt And Equity Securities [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	37		35
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	2,438		2,833
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | U S Treasury And Government [Member] | Other Fixed Maturity Investments [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	99		90
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | State and municipal bonds [Member] | Other Fixed Maturity Investments [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	230		728
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Foreign Government Debt [Member] | Other Fixed Maturity Investments [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	9		9
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Corporate Obligations [Member] | Other Fixed Maturity Investments [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	1,333		1,410
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member] | Mortgage-backed [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	310		219
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	21		28
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	26		47
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	130		121
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Other Asset Backed [Member] | Asset-backed [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	280		181
Fair Value Inputs Level 2 [Member] | Derivative Assets [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	2		2
Fair Value Inputs Level 2 [Member] | Assets Of Consolidated V I Es [Member] | Corporate Obligations [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	8		9
Fair Value Inputs Level 2 [Member] | Assets Of Consolidated V I Es [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	47		92
Fair Value Inputs Level 2 [Member] | Assets Of Consolidated V I Es [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	16		34
Fair Value Inputs Level 2 [Member] | Assets Of Consolidated V I Es [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	6		6
Fair Value Inputs Level 2 [Member] | Assets Of Consolidated V I Es [Member] | Other Asset Backed [Member] | Asset-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	11		10
Fair Value Inputs Level 2 [Member] | Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	2		2
Fair Value Inputs Level 2 [Member] | Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	152		157
Fair Value Inputs Level 2 [Member] | Liabilities Of Consolidated Vies [Member] | Variable Interest Entity Notes [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	124		114
Fair Value Inputs Level 3 [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	650		637
Fair value financial liabilities measured on recurring basis	498		513
Fair Value Inputs Level 3 [Member] | Medium-term Notes [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	106		102
Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	1		10
Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis			7
Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] | Other Asset Backed [Member] | Asset-backed [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	1		3
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Variable Interest Entity, Primary Beneficiary [Member] | Loan Repurchase Commitments [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	478		418
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Corporate Obligations [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis			5
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis			1
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Loans Receivable and Other Instruments At Fair Value [Member] | Variable Interest Entity, Primary Beneficiary [Member] | Residential Loans Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	146		172
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Currency Derivatives [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	8		17
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Other [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	17		14
Fair Value Inputs Level 3 [Member] | Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	10		33
Fair Value Inputs Level 3 [Member] | Derivative Liabilities [Member] | Other Derivatives [Member] | Non-Insured Derivatives [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	30		7
Fair Value Inputs Level 3 [Member] | Other Liabilities [Member] | Other Payable [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	4		5
Fair Value Inputs Level 3 [Member] | Liabilities Of Consolidated Vies [Member] | Variable Interest Entity Notes [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	$ 348		$ 366

[1] Investment that was measured at fair value by applying the net asset value per share practical expedient, and was required not to be classified in the fair value hierarchy.