Investments - Valuation changes included in net income for investments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2018
Investment [Line Items]
Total valuation changes	$ 122	$ 177
Equity securities
Investment [Line Items]
Total valuation changes	94	71
Limited partnership interests
Investment [Line Items]
Total valuation changes	$ 28	$ 106

Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

Name:	invest_InvestmentLineItems
Namespace Prefix:	invest_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration