Retirement Plans (Fair Value, Investments, Entities That Calculate Net Asset Value Per Share) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020		Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net Asset value per share, unfunded commitments	$ 500		$ 345
Investments, fair value disclosure	2,475		2,196
Hedge funds
Defined Benefit Plan Disclosure [Line Items]
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
Defined benefit plan, plan assets, amount	$ 1,112		$ 902
Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net Asset value per share, unfunded commitments	$ 290		$ 198
Fair value, investments, entities that calculate net asset value per share, investment redemption, frequency	(a)	[1]	(a)
Fair value, investments, entities that calculate net asset value per share, investment redemption, description	None		None
Defined benefit plan, plan assets, amount	$ 563		$ 522
Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net Asset value per share, unfunded commitments	$ 210		$ 147
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
Defined benefit plan, plan assets, amount	$ 800		$ 772

[1] A private equity fund investment ("partnership interest") is contractually locked up for the life of the private equity fund by the partnership agreement. Limited partners do not have the option to redeem partnership interests.