Reinsurance - GMIB Guarantees (Details) - Guarantee Type, Other [Member] - Guaranteed Minimum Income Benefit [Member] - Variable Annuity [Member] $ in Millions	Mar. 31, 2018 USD ($)
Guarantee Obligations [Line Items]
Underlying mutual fund investment values	$ 798
Maximum Exposure, Undiscounted	$ 568

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=82913245&loc=d3e12069-110248

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