Components of Deferred Tax Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Deferred tax assets:
Loss and loss expense reserves	$ 201	$ 182
Unearned premiums	113	107
Investments	36	31
Other	46	34
Total	396	354
Deferred tax liabilities:
Unrealized investment gains and losses	481	411
Deferred acquisition costs	167	157
Life policy reserves	42	26
Other	20	20
Total	710	614
Net deferred tax liability	$ 314	$ 260