Regulatory Capital - Summary of Capital Amounts, Tier 1 Risk Based and Tier 1 Leveraged Ratios (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Jan. 01, 2017
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity Tier 1 capital (to risk weighted assets), Actual Amount	$ 710,156		$ 689,494
Tier 1 capital (to risk weighted assets), Actual Amount	927,627		906,963
Total capital (to risk weighted assets), Actual Amount	1,047,698		1,021,601
Tier 1 capital (to average assets), Actual Amount	$ 927,627		$ 906,963
Common equity Tier 1 (to risk weighted assets), Actual Ratio	8.508%		8.805%
Tier 1 capital (to risk weighted assets), Actual Ratio	11.113%		11.583%
Total capital (to risk weighted assets), Actual Ratio	12.552%		13.047%
Tier 1 capital (to average assets), Actual Ratio	9.031%		8.937%
Common equity Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	$ 375,609		$ 352,368
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	500,812		469,824
Total capital (to risk weighted assets), For Capital Adequacy Purposes Amount	667,749		626,432
Tier 1 capital (to average assets), For Capital Adequacy Purposes Amount	$ 410,858		$ 405,949
Common equity Tier 1 (to risk weighted assets), For Capital Adequacy Purposes Ratio	4.50%	6.375%	4.50%
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	6.00%	7.875%	6.00%
Total capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	8.00%	9.875%	8.00%
Tier 1 capital (to average assets), For Capital Adequacy Purposes Ratio	4.00%		4.00%
Common equity Tier 1 (to risk weighted assets), for Basel III amount	$ 532,113		$ 450,248
Tier 1 (to risk weighted assets) Required for Basel III amount	657,316		567,704
Total capital (to risk weighted assets), for Basel III amount	824,253		724,313
Tier 1 (to risk average assets), for Basel III amount	$ 410,858		$ 405,949
Common equity Tier 1 (to risk weighted assets), for Basel III ratio	6.375%		5.75%
Tier 1 capital (to risk weighted assets), for Basel III ratio	7.875%		7.25%
Total capital (to risk weighted assets), for Basel III ratio	9.875%		9.25%
Tier 1 capital (to average assets), for Basel III ratio	4.00%		4.00%
Customers Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity Tier 1 capital (to risk weighted assets), Actual Amount	$ 1,037,480		$ 1,023,564
Tier 1 capital (to risk weighted assets), Actual Amount	1,037,480		1,023,564
Total capital (to risk weighted assets), Actual Amount	1,186,105		1,170,666
Tier 1 capital (to average assets), Actual Amount	$ 1,037,480		$ 1,023,564
Common equity Tier 1 (to risk weighted assets), Actual Ratio	12.448%		13.081%
Tier 1 capital (to risk weighted assets), Actual Ratio	12.448%		13.081%
Total capital (to risk weighted assets), Actual Ratio	14.231%		14.961%
Tier 1 capital (to average assets), Actual Ratio	10.107%		10.092%
Common equity Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	$ 375,048		$ 352,122
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	500,064		469,496
Total capital (to risk weighted assets), For Capital Adequacy Purposes Amount	666,751		625,994
Tier 1 capital (to average assets), For Capital Adequacy Purposes Amount	$ 410,612		$ 405,701
Common equity Tier 1 (to risk weighted assets), For Capital Adequacy Purposes Ratio	4.50%		4.50%	6.375%
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	6.00%		6.00%	7.875%
Total capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	8.00%		8.00%	9.875%
Tier 1 capital (to average assets), For Capital Adequacy Purposes Ratio	4.00%		4.00%
Common equity Tier 1 Capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 541,736		$ 508,621
Tier 1 capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	666,751		625,994
Total capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	833,439		782,493
Tier 1 capital (to average assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 513,265		$ 507,126
Common equity Tier 1 (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	6.50%		6.50%
Tier 1 capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	8.00%		8.00%
Total capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	10.00%		10.00%
Tier 1 capital (to average assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	5.00%		5.00%
Common equity Tier 1 (to risk weighted assets), for Basel III amount	$ 531,318		$ 449,934
Tier 1 (to risk weighted assets) Required for Basel III amount	656,333		567,307
Total capital (to risk weighted assets), for Basel III amount	823,021		723,806
Tier 1 (to risk average assets), for Basel III amount	$ 410,612		$ 405,701
Common equity Tier 1 (to risk weighted assets), for Basel III ratio	6.375%		5.75%
Tier 1 capital (to risk weighted assets), for Basel III ratio	7.875%		7.25%
Total capital (to risk weighted assets), for Basel III ratio	9.875%		9.25%
Tier 1 capital (to average assets), for Basel III ratio	4.00%		4.00%

Capital Required for Basel III

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Common Equity Tier One Capital Required for Basel III

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Tier One Common Capital Required for Basel III to Risk Weighted Assets

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Tier One Common Capital Required for Capital Adequacy to Risk Weighted Assets

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Tier One Common Capital Required to be Well Capitalized to Risk Weighted Assets

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Tier One Common Capital to Risk Weighted Assets

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Tier One Leverage Capital Required for Basel III

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Tier One Leverage Capital Required for Basel III to Average Assets

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Tier One Risk Based Capital Required for Basel III

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Tier One Risk Based Capital Required for Basel III 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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Amount of Common Equity Tier One Risk-based Capital, as defined by regulatory framework.

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Amount of Common Equity Tier 1 Risk-based Capital required to be categorized as adequately capitalized under regulatory framework for prompt corrective action.

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Amount of Common Equity Tier 1 Risk-based Capital required to be categorized as well-capitalized under regulatory framework for prompt corrective action.

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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.

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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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