Effect of Adopting ASU 2010-26 in Condensed Consolidated Balance Sheets (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified							9 Months Ended		12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010	Sep. 30, 2012 Accounting Standards Update 2010-26	Dec. 31, 2010 Scenario, Previously Reported	Dec. 31, 2011 Scenario, Previously Reported Accounting Standards Update 2010-26	Dec. 31, 2011 Scenario, As Adjusted Accounting Standards Update 2010-26	Dec. 31, 2011 Scenario, Difference Accounting Standards Update 2010-26
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred policy acquisition costs	$ 491	$ 484	$ 477	$ 479	$ 479	$ 458	$ 491		$ 510	$ 477	$ (33)
Total assets	16,479		15,635				16,479		15,668	15,635	(33)
Deferred income tax liability	453		303				453		314	303	(11)
Shareholders' equity	$ 5,359		$ 5,033	$ 4,764		$ 5,012	$ 5,359	$ 5,032	$ 5,055	$ 5,033	$ (22)
Book value per share							$ 32.95		$ 31.16	$ 31.03	$ (0.13)