Schedule I- Investments (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 4,118
Fair Value	4,072
Amount at which shown in the balance sheet	4,072
Assets Of Consolidated VIEs [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,192
Fair Value	1,254
Amount at which shown in the balance sheet	1,219
Other Aggregated Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1
Fair Value	1
Amount at which shown in the balance sheet	1
Loans Receivable [Member] | Assets Of Consolidated VIEs [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	159
Fair Value	172
Amount at which shown in the balance sheet	172
Long-Term Available-For-Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,618
Fair Value	3,582
Amount at which shown in the balance sheet	3,582
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	916
Fair Value	932
Amount at which shown in the balance sheet	932
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	641
Fair Value	727
Amount at which shown in the balance sheet	727
Long-Term Available-For-Sale [Member] | Foreign Governments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3
Fair Value	3
Amount at which shown in the balance sheet	3
Long-Term Available-For-Sale [Member] | Corporate Obligations [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,457
Fair Value	1,332
Amount at which shown in the balance sheet	1,332
Short-Term Available-For-Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	199
Fair Value	199
Amount at which shown in the balance sheet	199
Equity Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	0
Fair Value	0
Amount at which shown in the balance sheet	0
AFS Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,817
Fair Value	3,781
Amount at which shown in the balance sheet	3,781
Investments At Fair Value [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	300
Fair Value	290
Amount at which shown in the balance sheet	290
Investments At Fair Value [Member] | Assets Of Consolidated VIEs [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	143
Fair Value	157
Amount at which shown in the balance sheet	157
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	925
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	218
Fair Value	214
Amount at which shown in the balance sheet	214
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	30
Fair Value	27
Amount at which shown in the balance sheet	27
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	53
Fair Value	51
Amount at which shown in the balance sheet	51
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	122
Fair Value	119
Amount at which shown in the balance sheet	119
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	178
Fair Value	177
Amount at which shown in the balance sheet	$ 177