Claim and Claim Adjustment Expense Reserves (Impact of Loss Portfolio Transfer on the 2010 Consolidated Statement of Operations) (Details) (USD $) In Millions, 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
Other operating expenses									$ 1,335	$ 1,238	$ 1,787
Income tax (expense) benefit									(244)	(242)	(332)
Income (loss) from continuing operations	(9)	221	166	250	194	76	129	230	628	629	780
Income (loss) from discontinued operations								(1)		(1)	(21)
Net income (loss) attributable to CNA	(9)	221	166	250	193	75	124	220	628	612	691
Loss Portfolio Transfer [Member]
Other operating expenses											529
Income tax (expense) benefit											185
Income (loss) from continuing operations											(344)
Income (loss) from discontinued operations											(21)
Net income (loss) attributable to CNA											$ (365)