Debt (Schedule of debt instruments) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Long term debt	$ 2,710	$ 2,210
Total debt	2,710	2,560
Senior notes 6.950%, face amount $150, due January 15, 2018
Long term debt	150	150
Senior notes 7.350%, face amount $350, due November 15, 2019
Long term debt	349	349
Senior notes 5.875%, face amount $500, due August 15, 2020
Long term debt	498	497
Senior notes 5.750%, face amount $400, due August 15, 2021
Long term debt	398	397
Senior Notes, 3.950%, face amount of $550, due May 15, 2024
Long term debt	546	546
Senior Notes, 4.500%, face amount of $500, due March 1, 2026
Long term debt	498	0
Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023
Long term debt	241	241
Subordinated variable rate debt of Hardy, face amount of $30, due September 15, 2036
Long term debt	30	30
Senior notes 6.500%, face amount of $350, due August 15, 2016 [Member]
Short term debt	$ 0	$ 350