Regulatory Requirements (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Insurance [Abstract]
Permitted practice impact to statutory surplus	$ 377	$ (3)

Amount of each item listed to reconcile the differences as of the balance sheet date resulting from applying state permitted statutory accounting practices rather than statutory accounting practices permitted by National Association of Insurance Commissioners.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24746-158529

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