Schedule I- Investments (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 4,663
Fair Value	4,655
Amount at which shown in the balance sheet	4,655
Assets Of Consolidated VIEs [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,132
Fair Value	2,777
Amount at which shown in the balance sheet	2,751
Other Aggregated Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2
Fair Value	2
Amount at which shown in the balance sheet	2
Loans Receivable [Member] | Assets Of Consolidated VIEs [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,085
Fair Value	1,679
Amount at which shown in the balance sheet	1,679
Long-Term Available-For-Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,759
Fair Value	3,743
Amount at which shown in the balance sheet	3,743
Long-Term Available-For-Sale [Member] | U.S. Treasury And Government Agency [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	985
Fair Value	1,014
Amount at which shown in the balance sheet	1,014
Long-Term Available-For-Sale [Member] | US States And Political Subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	840
Fair Value	857
Amount at which shown in the balance sheet	857
Long-Term Available-For-Sale [Member] | Foreign Governments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4
Fair Value	4
Amount at which shown in the balance sheet	4
Long-Term Available-For-Sale [Member] | Corporate Obligations [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,173
Fair Value	1,117
Amount at which shown in the balance sheet	1,117
Short-Term Available-For-Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	676
Fair Value	676
Amount at which shown in the balance sheet	676
Equity Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3
Fair Value	4
Amount at which shown in the balance sheet	4
AFS Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4,438
Fair Value	4,423
Amount at which shown in the balance sheet	4,423
Investments At Fair Value [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	223
Fair Value	230
Amount at which shown in the balance sheet	230
Investments At Fair Value [Member] | Assets Of Consolidated VIEs [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	157
Fair Value	182
Amount at which shown in the balance sheet	182
Held-To-Maturity [Member] | Corporate Obligations [Member] | Assets Of Consolidated VIEs [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	890
Fair Value	916
Amount at which shown in the balance sheet	890
Mortgage-backed [Member] | Long-Term Available-For-Sale [Member] | Residential Mortgage Backed Agency [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	365
Fair Value	362
Amount at which shown in the balance sheet	362
Mortgage-backed [Member] | Long-Term Available-For-Sale [Member] | Residential Mortgage-Backed Non-Agency [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	35
Fair Value	32
Amount at which shown in the balance sheet	32
Mortgage-backed [Member] | Long-Term Available-For-Sale [Member] | Commercial Mortgage-Backed [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	66
Fair Value	66
Amount at which shown in the balance sheet	66
Asset-backed [Member] | Long-Term Available-For-Sale [Member] | Collateralized Debt Obligations [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	116
Fair Value	116
Amount at which shown in the balance sheet	116
Asset-backed [Member] | Long-Term Available-For-Sale [Member] | Other Asset-Backed [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	175
Fair Value	175
Amount at which shown in the balance sheet	$ 175