Schedule II. Condensed Financial Information of Registrant (Parent Company) (Schedule of Condensed Financial Information of Registrant, Statements of Operations and Comprehensive Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenues
Net realized investment gains (losses)									$ 31	$ 63	$ (2)
Other revenues									361	320	294
Total revenues	2,613	2,504	2,493	2,503	2,434	2,466	2,246	2,401	10,113	9,547	8,949
Expenses
Interest									166	170	175
Income tax expense (benefit)									376	244	242
Net income (loss) attributable to CNA	221	272	194	250	(9)	221	166	250	937	628	612
Total comprehensive income (loss) attributable to CNA									548	979	755
Parent Company [Member]
Revenues
Net investment income									1	1	1
Net realized investment gains (losses)									4	4	(9)
Other revenues										9	40
Total revenues									5	14	32
Expenses
Administrative and general									(7)	1	3
Interest									165	164	167
Total expenses									158	165	170
Income (loss) from operations before income taxes and equity in net income of subsidiaries									(153)	(151)	(138)
Income tax expense (benefit)									(22)	(144)	(46)
Income (loss) before equity in net income (loss) of subsidiaries									(131)	(7)	(92)
Equity in net income of subsidiaries									1,068	635	704
Net income (loss) attributable to CNA									937	628	612
Equity in other comprehensive income of subsidiaries									(389)	351	143
Total comprehensive income (loss) attributable to CNA									$ 548	$ 979	$ 755