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Debt (Schedule of debt instruments) (Details) - USD ($) | Dec. 31, 2017 | Dec. 31, 2016
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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

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