REGULATORY CAPITAL - Summary of Capital Amounts, Tier 1 Risk Based and Tier 1 Leveraged Ratios (Detail) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity Tier 1 capital (to risk weighted assets), Actual Amount	$ 1,777,683	$ 1,661,149
Tier 1 capital (to risk weighted assets), Actual Amount	1,915,476	1,798,942
Total capital (to risk weighted assets), Actual Amount	2,191,266	2,076,550
Tier 1 capital (to average assets), Actual Amount	$ 1,915,476	$ 1,798,942
Common equity Tier 1 (to risk weighted assets), Actual Ratio	12.463%	12.23%
Tier 1 capital (to risk weighted assets), Actual Ratio	0.13429	0.13245
Total capital (to risk weighted assets), Actual Ratio	0.15362	0.15289
Tier 1 capital (to average assets), Actual Ratio	0.08947	0.08375
Common equity Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	$ 641,885	$ 611,200
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	855,847	814,933
Total capital (to risk weighted assets), For Capital Adequacy Purposes Amount	1,141,129	1,086,578
Tier 1 capital (to average assets), For Capital Adequacy Purposes Amount	$ 856,386	$ 859,189
Common equity Tier 1 (to risk weighted assets), For Capital Adequacy Purposes Ratio	4.50%	4.50%
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	0.06000	0.06000
Total capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	0.08000	0.08000
Tier 1 capital (to average assets), For Capital Adequacy Purposes Ratio	0.04000	0.04000
Common equity Tier 1 (to risk weighted assets), for Basel III amount	$ 998,488	$ 950,755
Tier 1 (to risk weighted assets) Required for Basel III amount	1,212,450	1,154,489
Total capital (to risk weighted assets), for Basel III amount	1,497,732	1,426,133
Tier 1 (to risk average assets), for Basel III amount	$ 856,386	$ 859,189
Common equity Tier 1 (to risk weighted assets), for Basel III ratio	7.00%	7.00%
Tier 1 capital (to risk weighted assets), for Basel III ratio	8.50%	8.50%
Total capital (to risk weighted assets), for Basel III ratio	10.50%	10.50%
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	$ 1,943,059	$ 1,868,360
Tier 1 capital (to risk weighted assets), Actual Amount	1,943,059	1,868,360
Total capital (to risk weighted assets), Actual Amount	2,145,947	2,073,202
Tier 1 capital (to average assets), Actual Amount	$ 1,943,059	$ 1,868,360
Common equity Tier 1 (to risk weighted assets), Actual Ratio	13.636%	13.773%
Tier 1 capital (to risk weighted assets), Actual Ratio	0.13636	0.13773
Total capital (to risk weighted assets), Actual Ratio	0.15060	0.15283
Tier 1 capital (to average assets), Actual Ratio	0.09082	0.08708
Common equity Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	$ 641,212	$ 610,453
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	854,949	813,937
Total capital (to risk weighted assets), For Capital Adequacy Purposes Amount	1,139,932	1,085,250
Tier 1 capital (to average assets), For Capital Adequacy Purposes Amount	$ 855,807	$ 858,225
Common equity Tier 1 (to risk weighted assets), For Capital Adequacy Purposes Ratio	4.50%	4.50%
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	0.06000	0.06000
Total capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	0.08000	0.08000
Tier 1 capital (to average assets), For Capital Adequacy Purposes Ratio	0.04000	0.04000
Common equity Tier 1 Capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 926,125	$ 881,765
Tier 1 capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	1,139,932	1,085,250
Total capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	1,424,916	1,356,562
Tier 1 capital (to average assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 1,069,759	$ 1,072,782
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	0.08000	0.08000
Total capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	0.10000	0.10000
Tier 1 capital (to average assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	0.05000	0.05000
Common equity Tier 1 (to risk weighted assets), for Basel III amount	$ 997,441	$ 949,594
Tier 1 (to risk weighted assets) Required for Basel III amount	1,211,178	1,153,078
Total capital (to risk weighted assets), for Basel III amount	1,496,161	1,424,390
Tier 1 (to risk average assets), for Basel III amount	$ 855,807	$ 858,225
Common equity Tier 1 (to risk weighted assets), for Basel III ratio	7.00%	7.00%
Tier 1 capital (to risk weighted assets), for Basel III ratio	8.50%	8.50%
Total capital (to risk weighted assets), for Basel III ratio	10.50%	10.50%
Tier 1 capital (to average assets), for Basel III ratio	4.00%	4.00%