Investments - Gross realized gains and losses on available for sale fixed maturity and equity security investment transactions (Details) - Fixed maturities - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Gross realized gains and losses on the sale of available for sale fixed maturity and equity security investments
Gross gains	$ 6	$ 5
Gross losses	$ (3)	$ 0

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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