Fair Value Of Financial Instruments (Company's Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions		Mar. 31, 2019		Dec. 31, 2018
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis				$ 786
Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		$ 5,779		5,131
Fair value financial liabilities measured on recurring basis		1,633		786
Counterparty and cash collateral netting		(1)
Money Market Securities [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		91		67
Medium Term Notes [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis		106		102
Perpetual Debt And Equity Securities [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		63		58
Cash And Cash Equivalents [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		132		222
Fixed Income Funds [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	[1]	75		75
Fixed Maturities [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		3,870		3,871
Fixed Maturities [Member] | U S Treasury And Government [Member] | Other Fixed Maturity Investments [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		964		1,118
Fixed Maturities [Member] | State and municipal bonds [Member] | Other Fixed Maturity Investments [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		582		728
Fixed Maturities [Member] | Foreign Government Debt [Member] | Other Fixed Maturity Investments [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		13		9
Fixed Maturities [Member] | Corporate Obligations [Member] | Other Fixed Maturity Investments [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		1,689		1,410
Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member] | Mortgage-backed [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		229		219
Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		26		28
Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		55		54
Fixed Maturities [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		113		121
Fixed Maturities [Member] | Other Asset Backed [Member] | Asset-backed [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		199		184
Derivative Assets [Member] | Interest Rate Derivatives [Member] | Non Insured Derivatives [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		1		2
Assets Of Consolidated V I Es [Member] | Variable Interest Entity Primary Beneficiary [Member] | Loan Repurchase Commitments [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		420		418
Assets Of Consolidated V I Es [Member] | State and municipal bonds [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		707
Assets Of Consolidated V I Es [Member] | Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		14		14
Assets Of Consolidated V I Es [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		89		92
Assets Of Consolidated V I Es [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		33		34
Assets Of Consolidated V I Es [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		7		7
Assets Of Consolidated V I Es [Member] | Other Asset Backed [Member] | Asset-backed [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [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] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		206		172
Assets Of Consolidated V I Es [Member] | Currency Derivatives [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		14		17
Assets Of Consolidated V I Es [Member] | Cash And Cash Equivalents [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		31		58
Assets Of Consolidated V I Es [Member] | Other [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		15		14
Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis		21		35
Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Non Insured Derivatives [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis		171		157
Counterparty and cash collateral netting		(1)
Derivative Liabilities [Member] | Other Derivatives [Member] | Non Insured Derivatives [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis		7		7
Other Liabilities [Member] | Other Payable [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis		3		5
Liabilities Of Consolidated Vies [Member] | Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis		1,325		480
Fair Value Inputs Level 1 [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		1,060		1,331
Fair Value Inputs Level 1 [Member] | Money Market Securities [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		0		0
Fair Value Inputs Level 1 [Member] | Perpetual Debt And Equity Securities [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		26		23
Fair Value Inputs Level 1 [Member] | Cash And Cash Equivalents [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		132		222
Fair Value Inputs Level 1 [Member] | Fixed Maturities [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		871		1,028
Fair Value Inputs Level 1 [Member] | Fixed Maturities [Member] | U S Treasury And Government [Member] | Other Fixed Maturity Investments [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		871		1,028
Fair Value Inputs Level 1 [Member] | Fixed Maturities [Member] | Corporate Obligations [Member] | Other Fixed Maturity Investments [Member] | Fair Value Measurements Recurring [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] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		31		58
Fair Value Inputs Level 2 [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		3,884		3,021
Fair value financial liabilities measured on recurring basis		1,102		273
Fair Value Inputs Level 2 [Member] | Perpetual Debt And Equity Securities [Member] | Fair Value Measurements Recurring [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] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		2,991		2,833
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | U S Treasury And Government [Member] | Other Fixed Maturity Investments [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		93		90
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | State and municipal bonds [Member] | Other Fixed Maturity Investments [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		582		728
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Foreign Government Debt [Member] | Other Fixed Maturity Investments [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		13		9
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Corporate Obligations [Member] | Other Fixed Maturity Investments [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		1,689		1,410
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member] | Mortgage-backed [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		229		219
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		26		28
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		49		47
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		113		121
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Other Asset Backed [Member] | Asset-backed [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		197		181
Fair Value Inputs Level 2 [Member] | Derivative Assets [Member] | Interest Rate Derivatives [Member] | Non Insured Derivatives [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		1		2
Fair Value Inputs Level 2 [Member] | Assets Of Consolidated V I Es [Member] | State and municipal bonds [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		707
Fair Value Inputs Level 2 [Member] | Assets Of Consolidated V I Es [Member] | Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		9		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] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		89		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] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		33		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] | Fair Value Measurements Recurring [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] | Fair Value Measurements Recurring [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] | Fair Value Measurements Recurring [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] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis		172		157
Fair Value Inputs Level 2 [Member] | Liabilities Of Consolidated Vies [Member] | Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis		928		114
Fair Value Inputs Level 3 [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		669		637
Fair value financial liabilities measured on recurring basis		532		513
Fair Value Inputs Level 3 [Member] | Medium Term Notes [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	[2]	106		102
Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		8		10
Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	[2]	6		7
Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] | Other Asset Backed [Member] | Asset-backed [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	[2]	2		3
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Variable Interest Entity Primary Beneficiary [Member] | Loan Repurchase Commitments [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		420		418
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		5	[2]	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] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	[2]	1		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] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis		206		172
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Currency Derivatives [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	[2]	14		17
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Other [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial assets measured on recurring basis	[2]	15		14
Fair Value Inputs Level 3 [Member] | Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis		19		33
Fair Value Inputs Level 3 [Member] | Derivative Liabilities [Member] | Other Derivatives [Member] | Non Insured Derivatives [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis		7		7
Fair Value Inputs Level 3 [Member] | Other Liabilities [Member] | Other Payable [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis	[2]	3		5
Fair Value Inputs Level 3 [Member] | Liabilities Of Consolidated Vies [Member] | Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Recurring [Member]
Accounts Notes And Loans Receivable [Line Items]
Fair value financial liabilities measured on recurring basis		$ 397		$ 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.

[2] Unobservable inputs are either not developed by the Company or do not significantly impact the overall fair values of the aggregate financial assets and liabilities.