Employee Benefits - Fair Values of Postretirement Plan Assets by Asset Category (Details) - Other Postretirement Benefit Plans [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Plan assets	$ 52	$ 47	$ 52
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Defined Benefit Plan, Mutual Funds, Equity, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	22	21
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Equity - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	9	8
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Defined Benefit Plan, Mutual Funds, Fixed Income, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	7	7
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Defined Benefit Plan, Mutual Funds, Fixed Income, Non-US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets		0
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Defined Benefit Plan, Collective Investment Funds, Equity, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	12	9
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	38	36
Level 1 [Member] | Defined Benefit Plan, Mutual Funds, Equity, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	22	21
Level 1 [Member] | Equity - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	9	8
Level 1 [Member] | Defined Benefit Plan, Mutual Funds, Fixed Income, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	7	7
Level 1 [Member] | Defined Benefit Plan, Mutual Funds, Fixed Income, Non-US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets		0
Level 1 [Member] | Defined Benefit Plan, Collective Investment Funds, Equity, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 2 [Member] | Defined Benefit Plan, Mutual Funds, Equity, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 2 [Member] | Equity - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 2 [Member] | Defined Benefit Plan, Mutual Funds, Fixed Income, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 2 [Member] | Defined Benefit Plan, Mutual Funds, Fixed Income, Non-US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets		0
Level 2 [Member] | Defined Benefit Plan, Collective Investment Funds, Equity, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Defined Benefit Plan, Mutual Funds, Equity, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Equity - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Defined Benefit Plan, Mutual Funds, Fixed Income, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Defined Benefit Plan, Mutual Funds, Fixed Income, Non-US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets		0
Level 3 [Member] | Defined Benefit Plan, Collective Investment Funds, Equity, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Fair Value Measured at Net Asset Value Per Share [Member] | Other assets (measured at NAV) [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	$ 2	$ 2