Retirement Plans (Fair Value, Investments, Entities That Calculate Net Asset Value Per Share) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net Asset value per share, unfunded commitments	$ 345	$ 419
Investments, fair value disclosure	2,196	2,348
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	902	886
Fair value, investments, entities that calculate net Asset value per share, unfunded commitments	$ 0	$ 0
Fair value, investments, entities that calculate net asset value per share, investment redemption, frequency	Daily to annually	Daily to annually
Fair value, investments, entities that calculate net asset value per share, investment redemption, description	1 - 100 days	1 - 100 days
Private Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	$ 522	$ 518
Fair value, investments, entities that calculate net Asset value per share, unfunded commitments	$ 198	$ 310
Fair value, investments, entities that calculate net asset value per share, investment redemption, frequency	(a)	(a)
Fair value, investments, entities that calculate net asset value per share, investment redemption, description	None	None
Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	$ 772	$ 944
Fair value, investments, entities that calculate net Asset value per share, unfunded commitments	$ 147	$ 109
Fair value, investments, entities that calculate net asset value per share, investment redemption, frequency	Quarterly	Quarterly
Fair value, investments, entities that calculate net asset value per share, investment redemption, description	45 - 60 days	45 - 60 days