General (Narrative) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified		12 Months Ended			3 Months Ended
	Mar. 31, 2012 Basis of presentation [Member]	Dec. 31, 2011 CNA acquisition of Hardy common stock [Member]	Mar. 31, 2012 CNA acquisition of Hardy common stock [Member]	Dec. 31, 2011 Noncontrolling interest [Member]	Mar. 31, 2012 Accounting standards update 2010-26 [Member]	Mar. 31, 2011 Accounting standards update 2010-26 [Member]	Dec. 31, 2011 Accounting standards update 2010-26 [Member]
Noncontrolling interest, ownership percentage by parent	90.00%
Cash acquisition of common stock			$ 227
Gross written premiums written by Hardy		430
Restricted short-term investments held in escrow			230
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					2	3
Decrease in basic and diluted earnings per share					$ 0.01	$ 0.01
Decrease in deferred acquisition costs							106
Increase in deferred income taxes							37
Decrease in amortization of deferred acquisition costs						48
Increase in other operating expenses						52
Decrease in income tax expense						1
Change in net cash flows from operating, investing and financing activities