Fair Value Measurements - Postretirement Health Care Plan (Details) - Postretirement Health Care Benefits Plan - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	$ 160	$ 133	$ 151
Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	160	133
Recurring | Total investment securities
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	159	129
Recurring | Commingled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	84	74
Recurring | Government fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	24	12
Recurring | Corporate fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	43	34
Recurring | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	8	9
Recurring | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets		4
Recurring | Accrued income receivable
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	1
Level 1 | Recurring | Total investment securities
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	67	83
Level 1 | Recurring | Commingled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	59	74
Level 1 | Recurring | Government fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	0	0
Level 1 | Recurring | Corporate fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	0	0
Level 1 | Recurring | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	8	9
Level 2 | Recurring | Total investment securities
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	92	46
Level 2 | Recurring | Commingled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	25	0
Level 2 | Recurring | Government fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	24	12
Level 2 | Recurring | Corporate fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	43	34
Level 2 | Recurring | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	$ 0	$ 0