Other Liabilities And Provisions (Schedule Of Expected Future Payments And Sublease Recoveries) (Details) - Bow Office Building [Member] $ in Millions	Dec. 31, 2017 USD ($)
Other Liabilities And Provisions [Line Items]
Expected future payments, year one	$ 76
Expected future payments, year two	77
Expected future payments, year three	77
Expected future payments, year four	78
Expected future payments, year five	78
Expected future payments, Thereafter	1,295
Expected future payments, Total	1,681
Sublease recoveries, year one	(37)
Sublease recoveries, year two	(38)
Sublease recoveries, year three	(38)
Sublease recoveries, year four	(38)
Sublease recoveries, year five	(39)
Sublease recoveries, Thereafter	(636)
Sublease recoveries, Total	(826)
Expected Future Payments, Interest Amount [Member]
Other Liabilities And Provisions [Line Items]
Expected future payments, year one	65
Expected future payments, year two	65
Expected future payments, year three	63
Expected future payments, year four	63
Expected future payments, year five	62
Expected future payments, Thereafter	802
Expected future payments, amount representing interest	1,120
Present Value of Expected Future Payments [Member]
Other Liabilities And Provisions [Line Items]
Expected future payments, year one	11
Expected future payments, year two	12
Expected future payments, year three	14
Expected future payments, year four	15
Expected future payments, year five	16
Expected future payments, Thereafter	493
Expected future payments, present value, Total	$ 561