Employee Benefits - Fair Values of Postretirement Plan Assets by Asset Category (Details) - Other Postretirement Benefit Plans [Member] - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Plan assets	$ 47	$ 52	$ 50
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	21	25
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Equity - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	8	10
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	4
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		3
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	9	10
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	36	42
Level 1 [Member] | Defined Benefit Plan, Mutual Funds, Equity, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	21	25
Level 1 [Member] | Equity - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	8	10
Level 1 [Member] | Defined Benefit Plan, Mutual Funds, Fixed Income, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	7	4
Level 1 [Member] | Defined Benefit Plan, Mutual Funds, Fixed Income, Non-US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets		3
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