Net Capital Requirements - Summary of Capital Adequacy Requirements (Detail) $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Regulatory Capital Requirements [Abstract]
Total capital (to risk weighted assets), Actual, Amount	$ 740	$ 686
Total capital (to risk weighted assets), Actual, Ratio	184.6	137.7
Total capital (to risk weighted assets), For Capital Adequacy Purposes, Amount	$ 32	$ 40
Total capital (to risk weighted assets), For Capital Adequacy Purposes, Ratio	8.0	8.0
Total capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 40	$ 50
Total capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	10.0	10.0
Tier 1 capital (to risk weighted assets), Actual, Amount	$ 740	$ 686
Tier 1 capital (to risk weighted assets), Actual, Ratio	184.6	137.7
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes, Amount	$ 24	$ 30
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes, Ratio	6.0	6.0
Tier 1 capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 32	$ 40
Tier 1 capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.0	8.0
Tier 1 capital (to average assets), Actual, Amount	$ 740	$ 686
Tier 1 capital (to average assets), Actual, Ratio	71.3	72.8
Tier 1 capital (to average assets), For Capital Adequacy Purposes, Amount	$ 41	$ 38
Tier 1 capital (to average assets), For Capital Adequacy Purposes, Ratio	4.0	4.0
Tier 1 capital (to average assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 52	$ 47
Tier 1 capital (to average assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	5.0	5.0
Common Equity Tier 1 capital (to risk weighted assets), Actual, Amount	$ 740	$ 686
Common Equity Tier 1 capital (to risk weighted assets), Actual, Ratio	184.60%	137.70%
Common Equity Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes, Amount	$ 18	$ 22
Common Equity Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes, Ratio	4.50%	4.50%
Common Equity Tier 1 capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 26	$ 32
Common Equity Tier 1 capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%

Common equity tier one risk based capital.

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Amount of total capital as defined by regulatory framework.

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