Fair Value Of Financial Instruments (Quantitative Information Regarding The Significant Unobservable Inputs For Certain Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Cash Flows [Member] | Other Derivative Liabilities [Member] | Discounted cash flow [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value, liabilities		$ 7	$ 7
Cash Flows [Member] | Other Derivative Liabilities [Member] | Minimum [Member] | Discounted cash flow [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range dollars		0	0
Cash Flows [Member] | Other Derivative Liabilities [Member] | Maximum [Member] | Discounted cash flow [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range dollars		49	49
Cash Flows [Member] | Other Derivative Liabilities [Member] | Weighted Average [Member] | Discounted cash flow [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range dollars	[1]	25	25
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Direct Price Model [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value, liabilities		$ 19	$ 33
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Minimum [Member] | Direct Price Model [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage		54.00%	54.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Maximum [Member] | Direct Price Model [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage		54.00%	54.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Weighted Average [Member] | Direct Price Model [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage		54.00%	54.00%
Loans Receivable and Other Instruments at Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Quoted market prices adjusted for financial guarantees provided to VIE obligations and multiples of EBITDA [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, assets			$ 172
Loans Receivable and Other Instruments at Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Market prices adjusted for financial guarantees provided to VIE obligations [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, assets		$ 206
Loans Receivable and Other Instruments at Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Minimum [Member] | Quoted market prices adjusted for financial guarantees provided to VIE obligations and multiples of EBITDA [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage			(17.00%)
Loans Receivable and Other Instruments at Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Minimum [Member] | Market prices adjusted for financial guarantees provided to VIE obligations [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage		(9.00%)
Loans Receivable and Other Instruments at Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Maximum [Member] | Quoted market prices adjusted for financial guarantees provided to VIE obligations and multiples of EBITDA [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage			75.00%
Loans Receivable and Other Instruments at Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Maximum [Member] | Market prices adjusted for financial guarantees provided to VIE obligations [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage		50.00%
Loans Receivable and Other Instruments at Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Weighted Average [Member] | Quoted market prices adjusted for financial guarantees provided to VIE obligations and multiples of EBITDA [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage			7.00%
Loans Receivable and Other Instruments at Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Weighted Average [Member] | Market prices adjusted for financial guarantees provided to VIE obligations [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage		8.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity Primary Beneficiary [Member] | Recovery Rates And Breach Rates [Member] | Discounted cash flow [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, assets		$ 420	$ 418
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Quoted market prices of VIE assets adjusted for financial guarantees provided [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value, liabilities		$ 397	$ 366
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Minimum [Member] | Quoted market prices of VIE assets adjusted for financial guarantees provided [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage		0.00%	0.00%
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Maximum [Member] | Quoted market prices of VIE assets adjusted for financial guarantees provided [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage		65.00%	63.00%
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Weighted Average [Member] | Quoted market prices of VIE assets adjusted for financial guarantees provided [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage		41.00%	39.00%

[1] Midpoint of cash flows are used for the weighted average.