Investments (Schedule Of Available-For-Sale Securities Whose Value Is Below Amortized Cost) (Details) $ in Millions		Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Nine months or greater, but less than twelve months		$ 5,844	$ 17,813
Fair Value - Twelve months or greater		6,063	4,155
Fair Value - Total		11,907	21,968
Losses - Nine months or greater, but less than twelve months		89	659
Losses - Twelve months or greater		374	507
Losses - Total		$ 463	$ 1,166
Number of Securities - Total | security		1,128	1,744
Fair Value Decline, Greater Than 20% [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less than six months		$ 156	$ 174
Fair Value - Six months or greater, but less than nine months		2
Fair Value - Nine months or greater, but less than twelve months		15	1
Fair Value - Twelve months or greater		215	364
Fair Value - Total		388	539
Losses - Less than six months		57	52
Losses - Six months or greater, but less than nine months		1
Losses - Nine months or greater, but less than twelve months		8	1
Losses - Twelve months or greater		78	167
Losses - Total		144	220
OTTI - Less than six months		1	2
OTTI - Twelve months or greater		10	10
OTTI - Total		$ 11	$ 12
Number of Securities - Less than six months | security	[1]	26	19
Number of Securities - Six months or greater, but less than nine months | security	[1]	4
Number of Securities - Nine months or greater, but less than twelve months | security	[1]	7	2
Number of Securities - Twelve months or greater | security	[1]	49	62
Number of Securities - Total | security	[1]	86	83

[1] We may reflect a security in more than one aging category based on various purchase dates.