Investments (details) - OTTI Rollforward - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Roll-forward of the credit losses on fixed maturities recognized in the consolidated statement of income for which a portion of the other-than-temporary impairment was recognized in other comprehensive income (loss) [Roll Forward]
Cumulative OTTI credit losses recognized for securities held, beginning of period	$ 85	$ 83
Additions for OTTI securities where no credit losses were previously recognized		13
Additions for OTTI securities where credit losses have been previously recognized	1
Reductions due to sales/defaults of credit-impaired securities	(7)	(7)
Adjustments to book value of credit-impaired securities due to changes in cash flows	(4)	(4)
Cumulative OTTI credit losses recognized for securities still held, end of period	75	85
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Roll-forward of the credit losses on fixed maturities recognized in the consolidated statement of income for which a portion of the other-than-temporary impairment was recognized in other comprehensive income (loss) [Roll Forward]
Cumulative OTTI credit losses recognized for securities held, beginning of period	31	32
Adjustments to book value of credit-impaired securities due to changes in cash flows	(2)	(1)
Cumulative OTTI credit losses recognized for securities still held, end of period	29	31
All other corporate bonds [Member]
Roll-forward of the credit losses on fixed maturities recognized in the consolidated statement of income for which a portion of the other-than-temporary impairment was recognized in other comprehensive income (loss) [Roll Forward]
Cumulative OTTI credit losses recognized for securities held, beginning of period	54	51
Additions for OTTI securities where no credit losses were previously recognized		13
Additions for OTTI securities where credit losses have been previously recognized	1
Reductions due to sales/defaults of credit-impaired securities	(7)	(7)
Adjustments to book value of credit-impaired securities due to changes in cash flows	(2)	(3)
Cumulative OTTI credit losses recognized for securities still held, end of period	$ 46	$ 54

This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis.

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- Definition

This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which no such other than temporary impairment (OTTI) was previously recognized.

+ References

+ Details

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- Definition

The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563

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- Definition

This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held due to increases in cash flows expected to be collected and recognized over the remaining life of the security.

+ References

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- Definition

This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security because of the sale of impaired securities during the period.

+ References

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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