Derivative Financial Instruments (Effect Of Cash Flow Hedges On Financial Statements) (Details) - Forward Starting Pay Fixed Swaps [Member] - Cash Flow Hedging [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Gain (Loss) Recognized In Other Comprehensive Income (Loss)	$ 0	$ (9)	$ (33)	$ (109)
Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedge (loss) gain reclassified from accumulated other comprehensive loss	(4)	(2)	$ (10)	(5)
Net realized gains (losses) on financial instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedge (loss) gain reclassified from accumulated other comprehensive loss	$ 0	$ 0		$ (12)