RETIREMENT BENEFITS - NAV Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair Value	$ 1,173	$ 1,162
Unfunded Commitments	2
Domestic equity funds - large cap
Defined Benefit Plan Disclosure [Line Items]
Fair Value	394	339
Assets Valued at NAV Practical Expedient
Defined Benefit Plan Disclosure [Line Items]
Fair Value	680	674
Assets Valued at NAV Practical Expedient | Domestic equity funds - large cap
Defined Benefit Plan Disclosure [Line Items]
Fair Value	393	$ 318
Fair Value | Assets Valued at NAV Practical Expedient
Defined Benefit Plan Disclosure [Line Items]
Fair Value	152
Fair Value | Assets Valued at NAV Practical Expedient | Mutli-strategy hedge fund
Defined Benefit Plan Disclosure [Line Items]
Fair Value	0
Fair Value | Assets Valued at NAV Practical Expedient | Debt securities hedge fund
Defined Benefit Plan Disclosure [Line Items]
Fair Value	72
Fair Value | Assets Valued at NAV Practical Expedient | Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value	11
Fair Value | Assets Valued at NAV Practical Expedient | Domestic equity funds - large cap
Defined Benefit Plan Disclosure [Line Items]
Fair Value	69
Mutli-strategy hedge fund
Defined Benefit Plan Disclosure [Line Items]
Unfunded Commitments	$ 0
Redemption Notice Period	45 days
Debt securities hedge fund
Defined Benefit Plan Disclosure [Line Items]
Unfunded Commitments	$ 0
Redemption Notice Period	90 days
Private equity
Defined Benefit Plan Disclosure [Line Items]
Unfunded Commitments	$ 2
Domestic equity funds - large cap
Defined Benefit Plan Disclosure [Line Items]
Unfunded Commitments	$ 0