Segments of Operations - Narrative (Details) $ in Millions	3 Months Ended
Mar. 31, 2018 segment	Mar. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of segments \| segment	3
Property and Casualty Insurance \| Real Estate Investment
Segment Reporting Information [Line Items]
Pretax income from the sale of real estate properties \| $		$ 13

This element represents the amount of gain (loss) on sale of properties during the reporting period gross of the applicable income taxes realized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 974
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-15.(a)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=6875705&loc=d3e641549-123027

Name:	us-gaap_GainLossOnSaleOfPropertiesBeforeApplicableIncomeTaxes
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration