Consolidated Statements of Comprehensive Income (Loss) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Changes in:
Tax on net unrealized gains (losses) on investments with other-than-temporary impairments	$ (44)	$ (6)	$ (47)
Net unrealized gains (losses) on investments with other-than-temporary impairments	84	10	86
Tax on net unrealized gains (losses) on other investments	(183)	(203)	(269)
Net unrealized gains (losses) on other investments	341	372	505
Tax on net unrealized gains (losses) on investments	(227)	(209)	(316)
Net unrealized gains (losses) on investments	425	382	591
Tax on net unrealized gains (losses) on discontinued operations and other			(2)
Net unrealized gains (losses) on discontinued operations and other		(1)	9
Tax on foreign currency translation adjustment
Foreign currency translation adjustment	40	(15)	49
Tax on pension and postretirement benefits	61	111	(18)
Pension and postretirement benefits	(112)	(208)	35
Tax on allocation to participating policyholders
Allocation to participating policyholders	(2)	(7)	(23)
Tax on other comprehensive income (loss)	(166)	(98)	(336)
Other comprehensive income (loss), net of tax	351	151	661
Net income (loss)	628	628	759
Comprehensive income (loss)	979	779	1,420
Changes in:
Net unrealized (gains) losses on investments attributable to noncontrolling interests		(8)	(10)
Pension and postretirement benefits attributable to noncontrolling interests			(2)
Other comprehensive (income) loss attributable to noncontrolling interests		8	12
Net (income) loss attributable to noncontrolling interests		(16)	(68)
Comprehensive (income) loss attributable to noncontrolling interests		24	80
Total comprehensive income (loss) attributable to CNA	$ 979	$ 755	$ 1,340