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Investments - Net Investment Income and Investment Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Investment Income [Line Items]
Total investment income	$ 572	$ 575	$ 1,683	$ 1,613
Investment expenses	(15)	(14)	(44)	(43)
Investment gains	16	45	93	18
Net investment income	557	561	1,639	1,570
Fixed Maturity Securities [Member]
Net Investment Income [Line Items]
Total investment income	455	457	1,367	1,352
Investment gains	16	47	92	34
Equity Securities [Member]
Net Investment Income [Line Items]
Total investment income	1	1	4	8
Investment gains		(3)		(5)
Derivative Instruments [Member]
Net Investment Income [Line Items]
Investment gains	(1)	1	(3)	(12)
Short Term Investments [Member]
Net Investment Income [Line Items]
Total investment income	5	3	13	8
Investment gains	1		4	1
Limited Partnership Investments [Member]
Net Investment Income [Line Items]
Total investment income	67	91	206	98
Income from Trading Portfolio [Member]
Net Investment Income [Line Items]
Total investment income	34	11	67	113
Other [Member]
Net Investment Income [Line Items]
Total investment income	$ 10	$ 12	$ 26	$ 34

X

- Definition

Represents the net gain or loss, both realized and unrealized, on securities classified as trading and the total of interest and dividend income, including any amortization and accretion of discounts and premiums.

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- Definition

Net Investment Income And Gains Losses On Trading Securities Before Investment Expenses

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- Definition

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.

+ References

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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- Definition

Amount of expenses related to the generation of investment income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.2)
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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Line items represent financial 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.

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