Debt (Schedule of debt instruments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Short term debt:
Other debt	$ 13	$ 1
Total short term debt	83	400
Long term debt:
Debenture, 7.250%, face amount $243, due November 15, 2023	241	241
Other long-term debt		23
Total long term debt	2,525	2,251
Total	2,608	2,651
Senior notes 6.000%, face amount of $400, due August 15, 2011 [Member]
Short term debt:
Senior notes, current		399
Senior notes 8.375%, face amount of $70, due August 15, 2012 [Member]
Short term debt:
Senior notes, current	70
Long term debt:
Senior notes, noncurrent		69
Debenture - CNA Surety, face amount of $31, due April 29, 2034 [Member]
Long term debt:
Variable rate debt, noncurrent		31
Senior notes 5.850%, face amount $549, due December 15, 2014 [Member]
Long term debt:
Senior notes, noncurrent	548	548
Senior notes 6.500%, face amount of $350, due August 15, 2016 [Member]
Long term debt:
Senior notes, noncurrent	348	347
Senior notes 6.950%, face amount $150, due January 15, 2018 [Member]
Long term debt:
Senior notes, noncurrent	149	149
Senior notes 7.350%, face amount $350, due November 15, 2019 [Member]
Long term debt:
Senior notes, noncurrent	348	348
Senior notes 5.875%, face amount $500, due August 15, 2020 [Member]
Long term debt:
Senior notes, noncurrent	495	495
Senior notes 5.750%, face amount $400, due August 15, 2021 [Member]
Long term debt:
Senior notes, noncurrent	$ 396