Investments - Hybrid Securities (Details) - Equity securities [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Hybrid securities	$ 49	$ 36
Hybrid instruments, cost	$ 61	$ 49

Fair value as of the balance sheet date of hybrid instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 15
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6437212&loc=d3e50806-113969

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611

Name:	us-gaap_HybridInstrumentsAtFairValueNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant