REGULATORY MATTERS - Capital and Capital Ratio Information (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Common Equity Tier 1 to Risk-Weighted Assets (Amount)
Actual	[1]	$ 14,309	$ 13,822
Minimum Capital Adequacy	[1]	7,342	6,348
Classification as Well-capitalized	[1],[2]	$ 8,300	$ 8,051
Common Equity Tier 1 to Risk-Weighted Assets (Ratio)
Actual	[1]	11.20%	11.20%
Minimum Capital Adequacy	[1],[3]	5.75%	5.125%
Classification as Well-capitalized	[1],[2]	6.50%	6.50%
Tier 1 Capital to Risk-Weighted Assets (Amount)
Actual	[4]	$ 14,556	$ 14,069
Minimum Capital Adequacy	[4]	9,258	8,206
Classification as Well-capitalized	[2],[4]	$ 10,215	$ 9,909
Tier 1 Capital to Risk-Weighted Assets (Ratio)
Actual	[4]	11.40%	11.40%
Minimum Capital Adequacy	[3],[4]	7.25%	6.625%
Classification as Well-capitalized	[2],[4]	8.00%	8.00%
Total Capital to Risk-Weighted Assets (Amount)
Actual	[5]	$ 17,781	$ 17,347
Minimum Capital Adequacy	[5]	11,812	10,683
Classification as Well-capitalized	[2],[5]	$ 12,769	$ 12,386
Total Capital to Risk-Weighted Assets (Ratio)
Actual	[5]	13.90%	14.00%
Minimum Capital Adequacy	[3],[5]	9.25%	8.625%
Classification as Well-capitalized	[2],[5]	10.00%	10.00%
Tier 1 Capital to Average Assets (Leverage) (Amount)
Actual	[6]	$ 14,556	$ 14,069
Minimum Capital Adequacy	[6]	5,824	5,667
Classification as Well-capitalized	[2],[6]	$ 7,280	$ 7,084
Tier 1 Capital to Average Assets (Leverage) (Ratio)
Actual	[6]	10.00%	9.90%
Minimum Capital Adequacy	[3],[6]	4.00%	4.00%
Classification as Well-capitalized	[2],[6]	5.00%	5.00%
Capital conservation buffer		1.25%	0.625%

[1] “Common equity tier 1 capital ratio” represents CET1 capital divided by total risk-weighted assets as defined under U.S. Basel III Standardized approach.

[2] Presented for informational purposes. Prompt corrective action provisions apply only to the Company’s insured depository institutions - CBNA and CBPA.

[3] “Minimum Capital ratio” includes capital conservation buffer of 1.250% for 2017 and 0.625% for 2016; N/A to Tier 1 leverage.

[4] “Tier 1 capital ratio” is tier 1 capital, which includes CET1 capital plus non-cumulative perpetual preferred equity that qualifies as additional tier 1 capital, divided by total risk-weighted assets as defined under U.S. Basel III Standardized approach.

[5] “Total capital ratio” is total capital divided by total risk-weighted assets as defined under U.S. Basel III Standardized approach.

[6] “Tier 1 leverage ratio” is tier 1 capital divided by quarterly average total assets as defined under U.S. Basel III Standardized approach.