Investments (Net realized investment gains (losses)) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains (losses)	$ 93	$ 50	$ (67)
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	186	204	131
Gross realized losses	(64)	(138)	(197)
Net realized investment gains (losses)	122	66	(66)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	1	5	2
Gross realized losses	(1)	(10)	(25)
Net realized investment gains (losses)		(5)	(23)
Derivatives
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains (losses)	(4)	(2)	10
Short term investments and other
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains (losses)	$ (25)	$ (9)	$ 12

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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