General (Narrative) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Basis of presentation [Member]
Percentage owned of outstanding common stock by majority shareholder			90.00%
Noncontrolling interest [Member]
Sale of CNA ownership percentage of FICOH					50.00%
Accounting standards update 2010-26 [Member]
New accounting pronouncement or change in accounting principle, retrospective Application impracticable			Due to the lack of available historical data related to certain accident and health contracts issued prior to January 1, 2004, a full retrospective application of the change in accounting guidance was impracticable.
Decrease in net income attributable to CNA	$ 1	$ 2	$ 3	$ 5
Decrease in deferred acquisition costs					106
Increase in deferred income taxes					37
Decrease in amortization of deferred acquisition costs		64		112
Increase in other operating expenses		67		119
Changes in income tax expense benefit				1
Decrease in net income attributable to noncontrolling interest		1		1
Decrease in basic and diluted earnings per share	$ 0.01	$ 0.01	$ 0.01	$ 0.02
Change in net cash flows from operating, investing and financing activities