EARNINGS PER COMMON SHARE COMPUTATION - Details Supporting Computation of Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income available for common stockholders	$ 512	$ 689	$ 940	$ 566	$ 355	$ 644	$ 193	$ 491	$ 2,707	$ 1,683	$ 2,448
Weighted-average outstanding shares of common stock used to compute basic earnings per common share (in shares)									134,055	137,486	144,493
Shares used to compute diluted earnings per common share (in shares)									134,727	138,403	145,585
Basic earnings per common share (in USD per share)	$ 3.87	$ 5.16	$ 6.96	$ 4.18	$ 2.60	$ 4.68	$ 1.40	$ 3.56	$ 20.20	$ 12.24	$ 16.94
Diluted earnings per common share (in USD per share)	$ 3.84	$ 5.14	$ 6.94	$ 4.16	$ 2.58	$ 4.65	$ 1.39	$ 3.53	$ 20.10	$ 12.16	$ 16.81
Number of antidilutive stock options and restricted stock awards excluded from computation (in shares)									478	223	539
Employee stock options
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of employee stock options and restricted stock (in shares)									107	194	172
Restricted stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of employee stock options and restricted stock (in shares)									565	723	920