Regulatory Requirements (Regulatory Capital and Ratios) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
CSC [Member]
Common Equity Tier 1 Risk-Based Capital
Actual Amount	$ 15,535	$ 14,630
Actual Ratio	19.80%	19.30%
Minimum Capital Requirement Amount	$ 3,537	$ 3,414
Minimum Capital Requirement Ratio	4.50%	4.50%
Tier 1 Risk-Based Capital
Actual Amount	$ 18,328	$ 17,423
Actual Ratio	23.30%	23.00%
Minimum Capital Requirement Amount	$ 4,717	$ 4,552
Minimum Capital Requirement Ratio	6.00%	6.00%
Total Risk-Based Capital
Actual Amount	$ 18,372	$ 17,452
Actual Ratio	23.40%	23.00%
Minimum Capital Requirement Amount	$ 6,289	$ 6,069
Minimum Capital Requirement Ratio	8.00%	8.00%
Tier 1 Leverage
Actual Amount	$ 18,328	$ 17,423
Actual Ratio	7.50%	7.60%
Minimum Capital Requirement Amount	$ 9,832	$ 9,218
Minimum Capital Requirement Ratio	4.00%	4.00%
CSB [Member]
Common Equity Tier 1 Risk-Based Capital
Actual Amount	$ 14,093	$ 13,355
Actual Ratio	20.70%	20.10%
Minimum to be Well Capitalized Amount	$ 4,435	$ 4,324
Minimum to be Well Capitalized Ratio	6.50%	6.50%
Minimum Capital Requirement Amount	$ 3,070	$ 2,993
Minimum Capital Requirement Ratio	4.50%	4.50%
Tier 1 Risk-Based Capital
Actual Amount	$ 14,093	$ 13,355
Actual Ratio	20.70%	20.10%
Minimum to be Well Capitalized Amount	$ 5,458	$ 5,321
Minimum to be Well Capitalized Ratio	8.00%	8.00%
Minimum Capital Requirement Amount	$ 4,094	$ 3,991
Minimum Capital Requirement Ratio	6.00%	6.00%
Total Risk-Based Capital
Actual Amount	$ 14,121	$ 13,382
Actual Ratio	20.70%	20.10%
Minimum to be Well Capitalized Amount	$ 6,823	$ 6,652
Minimum to be Well Capitalized Ratio	10.00%	10.00%
Minimum Capital Requirement Amount	$ 5,458	$ 5,321
Minimum Capital Requirement Ratio	8.00%	8.00%
Tier 1 Leverage
Actual Amount	$ 14,093	$ 13,355
Actual Ratio	7.00%	7.10%
Minimum to be Well Capitalized Amount	$ 10,133	$ 9,462
Minimum to be Well Capitalized Ratio	5.00%	5.00%
Minimum Capital Requirement Amount	$ 8,107	$ 7,569
Minimum Capital Requirement Ratio	4.00%	4.00%

Common Equity Tier 1 Risk Based Capital as defined in the regulations.

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Common Equity Tier One Risk-Based Capital [Abstract]

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

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The minimum Common Equity Tier One Capital Ratio (Tier one 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 Common Equity Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The Common Equity Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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

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The amount of total capital as defined in the regulations.

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-Publisher FASB
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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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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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

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

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The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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

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