Employee Benefits - Fair Values of Postretirement Plan Assets by Asset Category (Details) - Other Postretirement Benefit Plans [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	$ 52	$ 50	$ 49
Mutual Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	25	23
Mutual Funds, Equity, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	10	5
Mutual Funds, Fixed Income, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	4	5
Mutual Funds, Fixed Income, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	3
Common Investment Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	10	10
Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value		7
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	42	33
Level 1 [Member] | Mutual Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	25	23
Level 1 [Member] | Mutual Funds, Equity, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	10	5
Level 1 [Member] | Mutual Funds, Fixed Income, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	4	5
Level 1 [Member] | Mutual Funds, Fixed Income, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	3
Level 1 [Member] | Common Investment Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 1 [Member] | Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value		0
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	10	17
Level 2 [Member] | Mutual Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Mutual Funds, Equity, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Mutual Funds, Fixed Income, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Mutual Funds, Fixed Income, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0
Level 2 [Member] | Common Investment Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	10	10
Level 2 [Member] | Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value		7
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Mutual Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Mutual Funds, Equity, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Mutual Funds, Fixed Income, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Mutual Funds, Fixed Income, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0
Level 3 [Member] | Common Investment Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	$ 0	0
Level 3 [Member] | Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value		$ 0