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Reinsurance (Components of Earned Premiums) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Effects of Reinsurance [Line Items] |  |  |
Direct premiums earned | $ 10,936 | $ 10,886 | $ 10,351
Assumed premiums earned | 367 | 308 | 324
Ceded premiums earned | 4,315 | 4,270 | 3,754
Net earned premiums | 6,988 | 6,924 | 6,921
Property and Casualty Insurance |  |  |
Effects of Reinsurance [Line Items] |  |  |
Direct premiums earned | 10,447 | 10,400 | 9,853
Assumed premiums earned | 317 | 258 | 274
Ceded premiums earned | 4,315 | 4,270 | 3,754
Net earned premiums | 6,449 | 6,388 | 6,373
Long term care Insurance Product Line |  |  |
Effects of Reinsurance [Line Items] |  |  |
Direct premiums earned | 489 | 486 | 498
Assumed premiums earned | 50 | 50 | 50
Ceded premiums earned | 0 | 0 | 0
Net earned premiums | $ 539 | $ 536 | $ 548
Percentage of assumed premiums earned to net premiums earned |  |  |
Effects of Reinsurance [Line Items] |  |  |
Percentage of assumed premium earned to premium earned net | 5.30% | 4.40% | 4.70%
Percentage of assumed premiums earned to net premiums earned | Property and Casualty Insurance |  |  |
Effects of Reinsurance [Line Items] |  |  |
Percentage of assumed premium earned to premium earned net | 4.90% | 4.00% | 4.30%
Percentage of assumed premiums earned to net premiums earned | Long term care Insurance Product Line |  |  |
Effects of Reinsurance [Line Items] |  |  |
Percentage of assumed premium earned to premium earned net | 9.30% | 9.30% | 9.10%

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- Definition Amount of earned premiums assumed from other entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition Amount of earned premiums ceded to other entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=84167274&loc=d3e27175-158546 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915 + Details
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- Definition Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums earned. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 + Details
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