Investments - Net realized investment gains (losses) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains (losses) on fixed maturity securities	$ 50	$ 16	$ 86	$ (20)
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	56	40	105	85
Gross realized losses	(12)	(24)	(29)	(86)
Net realized investment gains (losses) on fixed maturity securities	44	16	76	(1)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	1	4	1	4
Gross realized losses	(1)	(1)	(1)	(6)
Net realized investment gains (losses) on fixed maturity securities	0	3	0	(2)
Derivatives
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains (losses) on fixed maturity securities	(3)	(6)	(2)	(13)
Short term investments and other
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains (losses) on fixed maturity securities	$ 9	$ 3	$ 12	$ (4)

This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity 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

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