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Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Segment Reporting Information [Line Items] |  |
Reinsurance receivables | $ 4,290 | $ 4,453
Insurance receivables | 2,336 | 2,255
Deferred acquisition costs | 634 | 600
Goodwill | 148 | 145
Insurance reserves |  |
Claim and claim adjustment expenses | 22,004 | 22,343
Unearned premiums | 4,029 | 3,762
Future policy benefits | 11,179 | 10,326
Operating Segments | Specialty |  |
Segment Reporting Information [Line Items] |  |
Reinsurance receivables | 678 | 760
Insurance receivables | 984 | 982
Deferred acquisition costs | 319 | 310
Goodwill | 117 | 117
Insurance reserves |  |
Claim and claim adjustment expenses | 5,848 | 6,149
Unearned premiums | 2,035 | 1,911
Future policy benefits | 0 | 0
Operating Segments | Commercial |  |
Segment Reporting Information [Line Items] |  |
Reinsurance receivables | 647 | 621
Insurance receivables | 1,088 | 1,021
Deferred acquisition costs | 222 | 214
Goodwill | 0 | 0
Insurance reserves |  |
Claim and claim adjustment expenses | 8,585 | 8,894
Unearned premiums | 1,394 | 1,323
Future policy benefits | 0 | 0
Operating Segments | International |  |
Segment Reporting Information [Line Items] |  |
Reinsurance receivables | 212 | 131
Insurance receivables | 254 | 233
Deferred acquisition costs | 93 | 76
Goodwill | 31 | 28
Insurance reserves |  |
Claim and claim adjustment expenses | 1,636 | 1,328
Unearned premiums | 472 | 396
Future policy benefits | 0 | 0
Operating Segments | Life & Group |  |
Segment Reporting Information [Line Items] |  |
Reinsurance receivables | 438 | 462
Insurance receivables | 8 | 17
Deferred acquisition costs | 0 | 0
Goodwill | 0 | 0
Insurance reserves |  |
Claim and claim adjustment expenses | 3,499 | 3,358
Unearned premiums | 128 | 132
Future policy benefits | 11,179 | 10,326
Operating Segments | Corporate & Other |  |
Segment Reporting Information [Line Items] |  |
Reinsurance receivables | 2,315 | 2,479
Insurance receivables | 2 | 2
Deferred acquisition costs | 0 | 0
Goodwill | 0 | 0
Insurance reserves |  |
Claim and claim adjustment expenses | 2,436 | 2,614
Unearned premiums | 0 | 0
Future policy benefits | 0 | 0
Eliminations |  |
Segment Reporting Information [Line Items] |  |
Reinsurance receivables | 0 | 0
Insurance receivables | 0 | 0
Deferred acquisition costs | 0 | 0
Goodwill | 0 | 0
Insurance reserves |  |
Claim and claim adjustment expenses | 0 | 0
Unearned premiums | 0 | 0
Future policy benefits | $ 0 | $ 0

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