Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,600	$ 2,274
Net current-period other comprehensive income (loss)	8	(1)
Balance	1,535	2,396
Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(489)	(345)
Other comprehensive income (loss) before reclassifications		(4)
Amounts reclassified from accumulated other comprehensive loss to net loss	10	4
Net current-period other comprehensive income (loss)	10
Balance	(562)	(345)
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	26	(21)
Other comprehensive income (loss) before reclassifications
Amounts reclassified from accumulated other comprehensive loss to net loss	(2)	(1)
Net current-period other comprehensive income (loss)	(2)	(1)
Balance	28	(22)
Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred tax items	169	218	$ 250	$ 223
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(463)	(366)
Other comprehensive income (loss) before reclassifications		(4)
Amounts reclassified from accumulated other comprehensive loss to net loss	8	3
Net current-period other comprehensive income (loss)	8	(1)
Balance	(534)	$ (367)
Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Impact of adoption of ASU 2018-02	79
Accounting Standards Update 2018-02 [Member] | Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Impact of adoption of ASU 2018-02	(83)
Accounting Standards Update 2018-02 [Member] | Postretirement Benefits Other Than Pensions (OPEB) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Impact of adoption of ASU 2018-02	4
Accounting Standards Update 2018-02 [Member] | Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Impact of adoption of ASU 2018-02	$ (79)