REGULATORY MATTERS - Narrative (Details)	3 Months Ended		6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($) subsidiary shares	Mar. 31, 2016 USD ($)	Jun. 30, 2017 $ / shares	Dec. 31, 2016 $ / shares	Sep. 30, 2016 USD ($) subsidiary	Sep. 30, 2015 USD ($)	Jun. 30, 2017	Jul. 01, 2016 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Share repurchase plan, authorized amount								$ 690,000,000
Dividends	$ 62,000,000				$ 179,000,000
Dividends to preferred stockholders	$ 7,000,000	$ 7,000,000			14,000,000	$ 7,000,000
Treasury stock purchased (in shares) | shares	11,062,192
Treasury stock purchased	$ 250,000,000				$ 250,000,000	$ 500,000,000
Banking Subsidiaries
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Number of financial subsidiaries | subsidiary	2				2
CBNA subsidiaries
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Number of financial subsidiaries | subsidiary	2				2
Scenario, Forecast
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Dividends declared (in Dollars per Share) | $ / shares			$ 0.14	$ 0.12
Percentage increase in quarterly dividend							17.00%