Investments (Net realized investment gains (losses)) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains	$ 12	$ 36
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	69	49
Gross realized losses	(51)	(17)
Net realized investment gains	18	32
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized losses	(15)	0
Derivatives
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains	5	1
Short term investments and other
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains	$ 4	$ 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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