Net Capital Requirements - Summary of Capital Adequacy Requirements (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Regulatory Capital Requirements [Abstract]
Total capital (to risk weighted assets), Actual, Amount	$ 686	$ 572
Total capital (to risk weighted assets), Actual, Ratio	137.70%	131.10%
Total capital (to risk weighted assets), For Capital Adequacy Purposes, Amount	$ 40	$ 35
Total capital (to risk weighted assets), For Capital Adequacy Purposes, Ratio	8.00%	8.00%
Total capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 50	$ 44
Total capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%	10.00%
Tier 1 capital (to risk weighted assets), Actual, Amount	$ 686	$ 572
Tier 1 capital (to risk weighted assets), Actual, Ratio	137.70%	131.10%
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes, Amount	$ 30	$ 26
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes, Ratio	6.00%	6.00%
Tier 1 capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 40	$ 35
Tier 1 capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Tier 1 capital (to average assets), Actual, Amount	$ 686	$ 572
Tier 1 capital (to average assets), Actual, Ratio	72.80%	58.00%
Tier 1 capital (to average assets), For Capital Adequacy Purposes, Amount	$ 38	$ 39
Tier 1 capital (to average assets), For Capital Adequacy Purposes, Ratio	4.00%	4.00%
Tier 1 capital (to average assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 47	$ 49
Tier 1 capital (to average assets), To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%
Common Equity Tier 1 capital (to risk weighted assets), Actual, Amount	$ 686	$ 572
Common Equity Tier 1 capital (to risk weighted assets), Actual, Ratio	137.70%	131.10%
Common Equity Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes, Amount	$ 22	$ 20
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	$ 32	$ 28
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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Common equity tier one risk based capital required for capital adequacy.

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Common equity tier one risk based capital required for capital adequacy to risk weighted assets.

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Common equity tier one risk based capital required to be well capitalized.

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Common equity tier one risk based capital required to be well capitalized to risk weighted assets.

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Common equity tier one risk based capital to risk weighted assets.

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The amount of total capital as defined in the regulations.

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The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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The minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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