Investments (Mortgage Loans - Narrative) (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) Contracts	Dec. 31, 2016 USD ($) Contracts	Dec. 31, 2015 USD ($)
Financing Receivable, Recorded Investment, Aging [Abstract]
Financing Receivable, Significant Purchases	$ 420	$ 619
Percentage of Mortgage Loans Classified as Performing	99.00%	99.00%
Financing Receivable, Modifications, Subsequent Default, Number of Contracts \| Contracts	0	0
Agricultural
Financing Receivable, Recorded Investment, Aging [Abstract]
Estimated fair value of mortgage loans held-for-investment	$ 2,200	$ 1,900
Financing Receivable Number of Contract of Recorded Investment Past Due \| Contracts	0	0
Loans and Leases Receivable, Number of Contract, Nonperforming, Nonaccrual of Interest \| Contracts	0	0
Residential
Financing Receivable, Recorded Investment, Aging [Abstract]
Estimated fair value of mortgage loans held-for-investment	$ 1,200	$ 867
Financing Receivable Recorded Investment, Past Due	32	11
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	$ 11	$ 4
Commercial
Financing Receivable, Recorded Investment, Aging [Abstract]
Financing Receivable Number of Contract of Recorded Investment Past Due \| Contracts	0	0
Loans and Leases Receivable, Number of Contract, Nonperforming, Nonaccrual of Interest \| Contracts	0	0
Affiliated Entity [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Financing Receivable, Significant Purchases	$ 1,200	$ 2,400	$ 2,000
Related Party Transaction, Amounts of Transaction	$ 945	$ 1,600	$ 1,000

Reflects the number of financing receivables of recorded investment that are past due at the balance sheet date.

No definition available.

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- Definition

Reflects the number of loans deemed to be questionable as to collection on which no interest is continuing to be recognized.

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953682-111524

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- References

No definition available.

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- Definition

Recorded Investment in financing receivables that are past due at the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953401-111524

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- Definition

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

+ 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 (e)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

Reflects the carrying amount of loans deemed to be questionable as to collection on which no interest is continuing to be recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

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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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- Definition

Amount of transactions with related party during the financial reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39603-107864

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39622-107864

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