General (Narrative) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	9 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2012 Basis of presentation [Member]	Nov. 29, 2011 Noncontrolling interest [Member]	Sep. 30, 2011 Accounting standards update 2010-26 [Member]	Sep. 30, 2012 Accounting standards update 2010-26 [Member]	Sep. 30, 2011 Accounting standards update 2010-26 [Member]	Dec. 31, 2011 Accounting standards update 2010-26 [Member]
Percentage owned of outstanding common stock by majority shareholder	90.00%
Sale of CNA ownership percentage of FICOH		50.00%
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				$ 3	$ 5
Decrease in basic and diluted earnings per share				$ 0.01	$ 0.02
Decrease in deferred acquisition costs						106
Increase in deferred income taxes						37
Decrease in amortization of deferred acquisition costs			59		171
Increase in other operating expenses			59		178
Decrease in income tax expense			1		2
Increase in net income attributable to noncontrolling interest			1
Change in net cash flows from operating, investing and financing activities