Reinsurance (Components of Written Premiums) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Direct premiums written	$ 9,787us-gaap_DirectPremiumsWritten	$ 9,608us-gaap_DirectPremiumsWritten	$ 8,973us-gaap_DirectPremiumsWritten
Assumed premiums written	325us-gaap_AssumedPremiumsWritten	296us-gaap_AssumedPremiumsWritten	216us-gaap_AssumedPremiumsWritten
Ceded premiums written	3,024us-gaap_CededPremiumsWritten	2,556us-gaap_CededPremiumsWritten	2,225us-gaap_CededPremiumsWritten
Net written premiums	7,088us-gaap_PremiumsWrittenNet	7,348us-gaap_PremiumsWrittenNet	6,964us-gaap_PremiumsWrittenNet
Property and Casualty Insurance [Member]
Direct premiums written	9,283	9,103	8,467
Assumed premiums written	276	249	169
Ceded premiums written	3,024	2,556	2,225
Net written premiums	6,535	6,796	6,411
Accident and Health Insurance [Member]
Direct premiums written	504	505	506
Assumed premiums written	49	47	47
Ceded premiums written	0	0	0
Net written premiums	$ 553	$ 552	$ 553
Percentage of assumed premiums written to net premiums written [Member]
Percentage of assumed premium written to premium written net	4.60%	4.00%	3.10%
Percentage of assumed premiums written to net premiums written [Member] \| Property and Casualty Insurance [Member]
Percentage of assumed premium written to premium written net	4.20%	3.70%	2.60%
Percentage of assumed premiums written to net premiums written [Member] \| Accident and Health Insurance [Member]
Percentage of assumed premium written to premium written net	8.90%	8.50%	8.50%

Percentage of Assumed Premium Written to Premium Written Net

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- Definition

Amount of written premiums assumed from other entities.

+ 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)(2),(c)(2)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

+ Details

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- Definition

Amount of written premiums ceded to other entities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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

+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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)(1)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

+ Details

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- Details

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- Details

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