Employee Benefits - Fair Values of Pension Plan Assets by Asset Category (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	$ 1,046	$ 1,163	$ 1,133
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Government bonds - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	2	2
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan [Member] | Common - US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	6	11
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan [Member] | Common - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets		1
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan [Member] | Preferred - US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	3	3
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan [Member] | Corporate bonds - US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	157	152
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan [Member] | Corporate bonds - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	65	61
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan [Member] | Government and agency bonds - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	180	203
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan [Member] | State and municpal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	28	31
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan [Member] | Equity - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	1	7
Level 1 [Member] | Government bonds - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	10	22
Level 1 [Member] | Pension Plan [Member] | Common - US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	6	11
Level 1 [Member] | Pension Plan [Member] | Common - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets		1
Level 1 [Member] | Pension Plan [Member] | Preferred - US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	3	3
Level 1 [Member] | Pension Plan [Member] | Corporate bonds - US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 1 [Member] | Pension Plan [Member] | Corporate bonds - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 1 [Member] | Pension Plan [Member] | Government and agency bonds - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 1 [Member] | Pension Plan [Member] | State and municpal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 1 [Member] | Pension Plan [Member] | Equity - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	1	7
Level 2 [Member] | Government bonds - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	2	2
Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	432	449
Level 2 [Member] | Pension Plan [Member] | Common - US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 2 [Member] | Pension Plan [Member] | Common - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets		0
Level 2 [Member] | Pension Plan [Member] | Preferred - US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 2 [Member] | Pension Plan [Member] | Corporate bonds - US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	157	152
Level 2 [Member] | Pension Plan [Member] | Corporate bonds - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	65	61
Level 2 [Member] | Pension Plan [Member] | Government and agency bonds - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	180	203
Level 2 [Member] | Pension Plan [Member] | State and municpal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	28	31
Level 2 [Member] | Pension Plan [Member] | Equity - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Government bonds - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Pension Plan [Member] | Common - US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Pension Plan [Member] | Common - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets		0
Level 3 [Member] | Pension Plan [Member] | Preferred - US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Pension Plan [Member] | Corporate bonds - US [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Pension Plan [Member] | Corporate bonds - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Pension Plan [Member] | Government and agency bonds - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Pension Plan [Member] | State and municpal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Level 3 [Member] | Pension Plan [Member] | Equity - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Fair Value Measured at Net Asset Value Per Share [Member] | Pension Plan [Member] | Collective Investment Funds (measured at NAV) [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	546	628
Fair Value Measured at Net Asset Value Per Share [Member] | Pension Plan [Member] | Insurance investment contracts and pooled separate accounts (measured at NAV) [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	16	14
Fair Value Measured at Net Asset Value Per Share [Member] | Pension Plan [Member] | Other assets (measured at NAV) [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	$ 42	$ 50