Summary Of Significant Accounting Policies (Schedule Of Effect Of Adopting ASU 2010-26) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Deferred policy acquisition costs	$ 470				$ 477				$ 470	$ 477	$ 458	$ 454
Total assets	16,548				15,635				16,548	15,635
Deferred income tax	453				303				453	303
Shareholders' equity	5,453				5,033				5,453	5,033	5,012	4,760
Book value per share									$ 33.48
Underwriting acquisition and insurance									1,155	1,039	1,024
Net income	192	111	32	86	134	19	(50)	61	421	164	375
Basic	$ 1.18	$ 0.69	$ 0.20	$ 0.53	$ 0.82	$ 0.12	$ (0.31)	$ 0.38	$ 2.59	$ 1.01	$ 2.30
Diluted	$ 1.17	$ 0.68	$ 0.20	$ 0.53	$ 0.82	$ 0.12	$ (0.31)	$ 0.38	$ 2.57	$ 1.01	$ 2.30
As Originally Reported [Member]
Deferred policy acquisition costs					510					510
Total assets					15,668					15,668
Deferred income tax					314					314
Shareholders' equity					5,055					5,055
Book value per share										$ 31.16
Underwriting acquisition and insurance										1,036	1,021
Net income										166	377
Basic										$ 1.02	$ 2.32
Diluted										$ 1.02	$ 2.31
As Adjusted [Member]
Deferred policy acquisition costs					477					477
Total assets					15,635					15,635
Deferred income tax					303					303
Shareholders' equity					5,033					5,033		4,742
Book value per share										$ 31.03
Underwriting acquisition and insurance										1,039	1,024
Net income										164	375
Basic										$ 1.01	$ 2.30
Diluted										$ 1.01	$ 2.30
Difference [Member]
Deferred policy acquisition costs					(33)					(33)
Total assets					(33)					(33)
Deferred income tax					(11)					(11)
Shareholders' equity					(22)					(22)
Book value per share										$ (0.13)
Underwriting acquisition and insurance										3	3
Net income										$ (2)	$ (2)
Basic										$ (0.01)	$ (0.02)
Diluted										$ (0.01)	$ (0.01)