REGULATORY MATTERS - Narrative (Details) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 $ / shares	Dec. 31, 2017 USD ($) subsidiary $ / shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Revaluation of deferred tax liability - net tax benefit from 2017 Tax Legislation		$ 331
Revaluation of deferred tax asset - net tax expense from 2017 Tax Legislation		145
Reclassification of tax effects resulting from the 2017 Tax Legislation		$ 145
Common stock dividends (in dollars per share) | $ / shares		$ 0.18
Dividends		$ 322	$ 241
Treasury stock purchased		820	430	$ 500
Repayments of long-term borrowed funds		12,751	8,429	766
Preferred stock dividends		$ 14	14	$ 7
Subordinated Debt
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Repayments of long-term borrowed funds			$ 625
Forecast
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock dividends (in dollars per share) | $ / shares	$ 0.22
Bank subsidiaries
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Number of financial subsidiaries | subsidiary		2