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Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Specialty - USD ($) $ in Millions | 12 Months Ended | 24 Months Ended | 36 Months Ended | 48 Months Ended | 60 Months Ended | 72 Months Ended | 84 Months Ended | 96 Months Ended | 108 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net prior year claim and allocated claim adjustment expense reserve development, Surety | $ (84) | $ (63) | $ (69) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Surety |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (82) | (65) | (65) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total net development for accident years prior to 2008 | 1 | 2 | (4) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total unallocated claim adjustment expense development | (3) | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net prior year claim and allocated claim adjustment expense reserve development, Surety | (84) | (63) | (69) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Surety | 2008 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | (1) | (3) | $ (4) | $ (9) | $ (7) | $ (5) | $ (41) | $ 0 |  |  |  |  |  |  |  | $ (70)
Surety | 2009 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | 1 | (7) | (9) | (17) | (18) | (11) | $ 0 |  |  |  |  |  |  |  | $ (61) |
Surety | 2010 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (4) | (3) | (10) | (8) | (27) | (1) | $ 0 |  |  |  |  |  |  |  | $ (53) |  |
Surety | 2011 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (4) | (5) | (12) | (29) | (5) | $ 1 |  |  |  |  |  |  |  | $ (54) |  |  |
Surety | 2012 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (7) | (18) | (28) | (24) | $ 2 |  |  |  |  |  |  |  | $ (75) |  |  |  |
Surety | 2013 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (15) | (9) | (6) | $ 1 |  |  |  |  |  |  |  | $ (29) |  |  |  |  |
Surety | 2014 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (25) | (30) | $ 1 |  |  |  |  |  |  |  | $ (54) |  |  |  |  |  |
Surety | 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (27) | 0 |  |  |  |  |  |  |  | $ (27) |  |  |  |  |  |  |
Surety | 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | $ 0 | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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