FAIR VALUE MEASUREMENTS - Schedule of Financial Instruments not Recorded at Fair Value (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities held to maturity, carrying value		$ 4,555	$ 4,685
Securities held-to-maturity, fair value		4,439	4,668
Equity securities, at cost, carrying value		748	722
Equity securities, at cost, fair value		748	722
Other loans held for sale, carrying value		322	221
Other loans held for sale, fair value		322	221
Loans and leases, carrying value	[1],[2]	111,425	110,617
Loans and leases, fair value		110,756	111,168
Deposits, carrying value		115,730	115,089
Deposits, fair value		115,563	115,039
Federal funds purchased and securities sold under agreements to repurchase, carrying value		315	815
Federal funds purchased and securities sold under agreements to repurchase, fair value		315	815
Other short-term borrowed funds, carrying value		1,494	1,856
Other short-term borrowed funds, fair value		1,494	1,856
Long-term borrowed funds, carrying value		13,486	11,765
Long-term borrowed funds, fair value		13,525	11,891
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities held to maturity, carrying value		0	0
Securities held-to-maturity, fair value		0	0
Equity securities, at cost, carrying value		0	0
Equity securities, at cost, fair value		0	0
Other loans held for sale, carrying value		0	0
Other loans held for sale, fair value		0	0
Loans and leases, carrying value		0	0
Loans and leases, fair value		0	0
Deposits, carrying value		0	0
Deposits, fair value		0	0
Federal funds purchased and securities sold under agreements to repurchase, carrying value		0	0
Federal funds purchased and securities sold under agreements to repurchase, fair value		0	0
Other short-term borrowed funds, carrying value		0	0
Other short-term borrowed funds, fair value		0	0
Long-term borrowed funds, carrying value		0	0
Long-term borrowed funds, fair value		0	0
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities held to maturity, carrying value		4,555	4,685
Securities held-to-maturity, fair value		4,439	4,668
Equity securities, at cost, carrying value		748	722
Equity securities, at cost, fair value		748	722
Other loans held for sale, carrying value		0	0
Other loans held for sale, fair value		0	0
Loans and leases, carrying value		418	393
Loans and leases, fair value		418	393
Deposits, carrying value		115,730	115,089
Deposits, fair value		115,563	115,039
Federal funds purchased and securities sold under agreements to repurchase, carrying value		315	815
Federal funds purchased and securities sold under agreements to repurchase, fair value		315	815
Other short-term borrowed funds, carrying value		1,494	1,856
Other short-term borrowed funds, fair value		1,494	1,856
Long-term borrowed funds, carrying value		13,486	11,765
Long-term borrowed funds, fair value		13,525	11,891
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities held to maturity, carrying value		0	0
Securities held-to-maturity, fair value		0	0
Equity securities, at cost, carrying value		0	0
Equity securities, at cost, fair value		0	0
Other loans held for sale, carrying value		322	221
Other loans held for sale, fair value		322	221
Loans and leases, carrying value		111,007	110,224
Loans and leases, fair value		110,338	110,775
Deposits, carrying value		0	0
Deposits, fair value		0	0
Federal funds purchased and securities sold under agreements to repurchase, carrying value		0	0
Federal funds purchased and securities sold under agreements to repurchase, fair value		0	0
Other short-term borrowed funds, carrying value		0	0
Other short-term borrowed funds, fair value		0	0
Long-term borrowed funds, carrying value		0	0
Long-term borrowed funds, fair value		$ 0	$ 0

[1] Excluded from the table above are loans held for sale totaling $800 million and $718 million as of March 31, 2018 and December 31, 2017, respectively.

[2] Mortgage loans serviced for others by the Company’s subsidiaries are not included above, and amounted to $20.2 billion and $20.3 billion at March 31, 2018 and December 31, 2017, respectively.