Investments - Net realized investment gains (losses) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Net realized investment (losses) gains	$ (24)	$ 46	$ 62	$ 26
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	34	67	139	152
Gross realized losses	(18)	(20)	(47)	(106)
Net realized investment (losses) gains	16	47	92	46
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	0	1	1	5
Gross realized losses	0	(4)	(1)	(10)
Net realized investment (losses) gains	0	(3)	0	(5)
Derivatives
Schedule of Available-for-sale Securities [Line Items]
Net realized investment (losses) gains	(1)	1	(3)	(12)
Short term investments and other
Schedule of Available-for-sale Securities [Line Items]
Net realized investment (losses) gains	$ (39)	$ 1	$ (27)	$ (3)

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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