REGULATORY MATTERS - Narrative (Details)					6 Months Ended		12 Months Ended
	Jan. 20, 2017 $ / shares	Jul. 28, 2016 USD ($)	Aug. 03, 2015 shares	Apr. 07, 2015 shares	Jun. 30, 2017 $ / shares	Dec. 31, 2016 USD ($) subsidiary $ / shares	Dec. 31, 2016 USD ($) subsidiary shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)	Jan. 30, 2017 USD ($)	Jul. 01, 2016 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock dividends (in dollars per share) | $ / shares						$ 0.12
Share repurchase plan, authorized amount											$ 690,000,000
Classification under final rule - large non-complex firm total assets						$ 149,520,000,000	$ 149,520,000,000	$ 138,208,000,000
Dividends							$ 241,000,000	$ 214,000,000
Treasury stock purchased (in shares) | shares			9,615,384	10,473,397			17,300,000	20,100,000
Repayments of long-term borrowed funds		$ 500,000,000					$ 8,429,000,000	$ 766,000,000	$ 6,000,000
Preferred stock dividends							$ 14,000,000	7,000,000
Bank subsidiaries
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Number of financial subsidiaries | subsidiary						2	2
Forecast
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock dividends (in dollars per share) | $ / shares					$ 0.14
Increase to common dividends					17.00%
Subordinated Debt
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Repayments of long-term borrowed funds							$ 625,000,000	$ 750,000,000
2016 CCAR Plan
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock purchased (in shares) | shares							17,332,684
Subsequent Event
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock dividends (in dollars per share) | $ / shares	$ 0.14
Subsequent Event | Minimum | 2016 CCAR Plan
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Classification under final rule - large non-complex firm total assets										$ 50,000,000,000
Subsequent Event | Maximum | 2016 CCAR Plan
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Classification under final rule - large non-complex firm total assets										250,000,000,000
Subsequent Event | Maximum | 2016 CCAR Plan | Nonbank subsidiaries
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Classification under final rule - large non-complex firm total assets										$ 75,000,000,000