Commitments, Guarantees and Contingent Liabilities - Texas Associations (Details) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies
Liability for claims	$ 26,325,000,000	$ 25,250,000,000
Texas Windstorm Insurance Association
Loss Contingencies
Issuance of securities to participating insurers upon utilization of funds set aside	$ 1,000,000,000
Participation ratio	13.60%
Texas Fair Plan Association
Loss Contingencies
Liability for claims	$ 8,000,000

The amount needed to reflect the estimated ultimate cost of settling claims relating to casualty insurance insured events that have occurred on or before a particular date (ordinarily, the balance sheet date) and the amount needed to provide for the estimated ultimate cost required to investigate and settle claims relating to insured events that have occurred on or before a particular date (ordinarily, the balance sheet date), whether or not reported to the insurer at that date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-18(Column C))
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant