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Debt (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended 11 Months Ended 2 Months Ended
Dec. 31, 2011
Feb. 01, 2011
Senior notes 5.750%, face amount $400, due August 15, 2021 [Member]
Dec. 31, 2011
Extinguished senior notes, due August 15, 2011 [Member]
Feb. 01, 2011
Extinguished senior notes, due August 15, 2011 [Member]
Dec. 31, 2011
Extinguished variable rate debt, Debenture - CNA Surety, due April 29, 2034 [Member]
Senior notes issued   $ 400      
Stated interest rate on senior notes   5.75%   6.00%  
Redemption of debt     400   31
Line of credit facility, initiation date August 1, 2007        
Line of credit facility, expiration date August 1, 2012        
Maximum borrowing capacity under revolving credit facilities 250        
Debt instrument, description of variable rate basis London Interbank Offered Rate (LIBOR) plus the Company's credit risk spread        
Limit available for credit agreement $ 250