Debt (Schedule of debt instruments) (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long term debt	$ 2,708,000,000	$ 2,710,000,000
Total	$ 2,858,000,000	2,710,000,000
Senior notes 6.500%, face amount of $350, due August 15, 2016
Debt Instrument [Line Items]
Stated interest rate	6.95%
Face amount	$ 150,000,000
Short term debt	$ 150,000,000	0
Senior notes 6.950%, face amount $150, due January 15, 2018
Debt Instrument [Line Items]
Stated interest rate	6.95%
Face amount	$ 150,000,000
Long term debt	$ 0	150,000,000
Senior notes 7.350%, face amount $350, due November 15, 2019
Debt Instrument [Line Items]
Stated interest rate	7.35%
Face amount	$ 350,000,000
Long term debt	$ 0	349,000,000
Senior notes 5.875%, face amount $500, due August 15, 2020
Debt Instrument [Line Items]
Stated interest rate	5.875%
Face amount	$ 500,000,000
Long term debt	$ 498,000,000	498,000,000
Senior notes 5.750%, face amount $400, due August 15, 2021
Debt Instrument [Line Items]
Stated interest rate	5.75%
Face amount	$ 400,000,000
Long term debt	$ 398,000,000	398,000,000
Senior Notes, 3.950%, face amount of $550, due May 15, 2024
Debt Instrument [Line Items]
Stated interest rate	3.95%
Face amount	$ 550,000,000
Long term debt	$ 547,000,000	546,000,000
Senior Notes, 4.500%, face amount of $500, due March 1, 2026
Debt Instrument [Line Items]
Stated interest rate	4.50%
Face amount	$ 500,000,000
Long term debt	$ 498,000,000	498,000,000
Senior Notes, 3.450%, face amount of $500, due August 15, 2027
Debt Instrument [Line Items]
Stated interest rate	3.45%
Face amount	$ 500,000,000
Long term debt	$ 495,000,000	0
Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023
Debt Instrument [Line Items]
Stated interest rate	7.25%
Face amount	$ 243,000,000
Long term debt	242,000,000	241,000,000
Subordinated variable rate debt of Hardy, face amount of $30, due September 15, 2036
Debt Instrument [Line Items]
Face amount	30,000,000
Long term debt	$ 30,000,000	$ 30,000,000

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
-Section S99
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