Additional Shareholders' Equity Information - Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Loss
Beginning Balance	$ 10,619	$ 9,600	$ 10,370
Realized Amounts Reclassified Into Earnings	(2)	5	5
Unrealized Change in Fair Value	0	(4)	(3)
Ending Balance	10,484	10,619	9,600
Total
Accumulated Other Comprehensive Loss
Beginning Balance	(30)	(31)	(33)
Ending Balance	(32)	(30)	(31)
Interest Rate Cash Flow Hedge
Accumulated Other Comprehensive Loss
Beginning Balance	(20)	(21)	(22)
Realized Amounts Reclassified Into Earnings	(3)	1	1
Unrealized Change in Fair Value	0	0	0
Ending Balance	(23)	(20)	(21)
Other Postretirement Benefit Plans
Accumulated Other Comprehensive Loss
Beginning Balance	(10)	(10)	(11)
Realized Amounts Reclassified Into Earnings	1	4	4
Unrealized Change in Fair Value	0	(4)	(3)
Ending Balance	$ (9)	$ (10)	$ (10)