STOCK-BASED COMPENSATION - Activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted stock units
Number of Shares
Non-vested at beginning of year (in shares)	3,068,168
Granted (in shares)	1,031,837
Vested (in shares)	(1,588,319)
Forfeited (in shares)	(338,315)
Non-vested at end of year (in shares)	2,173,371	3,068,168
Weighted Average Grant-Date Fair Value
Nonvested at beginning of year (in dollars per share)	$ 35.51
Granted (in dollars per share)	52.35
Vested (in dollars per share)	36.21
Forfeited (in dollars per share)	40.50
Nonvested at end of year (in dollars per share)	$ 42.22	$ 35.51
Other information
Total intrinsic value, vested shares (in dollars)	$ 81	$ 60	$ 46
Performance leveraged stock units
Number of Shares
Non-vested at beginning of year (in shares)	1,953,797
Granted (in shares)	693,714
Vested (in shares)	(881,656)
Forfeited (in shares)	(378,574)
Non-vested at end of year (in shares)	1,387,281	1,953,797
Weighted Average Grant-Date Fair Value
Nonvested at beginning of year (in dollars per share)	$ 44.46
Granted (in dollars per share)	58.28
Vested (in dollars per share)	48.27
Forfeited (in dollars per share)	43.69
Nonvested at end of year (in dollars per share)	$ 49.16	$ 44.46
Other information
Total intrinsic value, vested shares (in dollars)	$ 42	$ 71	$ 68