Investments (Mortgage Loans - Narrative) (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) Contracts	Dec. 31, 2016 USD ($) Contracts	Dec. 31, 2015 USD ($)
Mortgage Loans on Real Estate [Line Items]
Allowance for Loan and Lease Losses, Write-offs	$ 14	$ 160	$ 35
Financing Receivable, Significant Purchases	3,100	2,900
Average Recorded Investment	$ 322	$ 327
Percentage of Mortgage Loans Classified as Performing	99.00%	99.00%
Agricultural
Mortgage Loans on Real Estate [Line Items]
Allowance for Loan and Lease Losses, Write-offs	$ 2	$ 1	0
Average Recorded Investment	32	49	60
Estimated fair value of mortgage loans held-for-investment	13,100	12,700
Residential
Mortgage Loans on Real Estate [Line Items]
Allowance for Loan and Lease Losses, Write-offs	12	16	16
Average Recorded Investment	285	188	84
Estimated fair value of mortgage loans held-for-investment	11,600	11,200
Commercial mortgage loans
Mortgage Loans on Real Estate [Line Items]
Allowance for Loan and Lease Losses, Write-offs	0	143	19
Average Recorded Investment	$ 5	$ 90	$ 127
Commercial Portfolio Segment [Member]
Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Modifications, Number of Contracts \| Contracts	0	0
Residential
Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Modifications, Number of Contracts \| Contracts	500	557
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 120	$ 136
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 108	$ 122

The percentage of all mortgage loans that are classified as performing.

No definition available.

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- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

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- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

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- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

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- Definition

The amount of any significant purchases of financing receivables during the reporting period.

+ References

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- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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