Investments (Mortgage Loans - Narrative) (Details) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018 USD ($) Contracts	Mar. 31, 2017 Contracts	Dec. 31, 2017 USD ($) Contracts
Financing Receivable, Recorded Investment, Aging [Abstract]
Financing Receivable, Significant Purchases \| $	$ 86		$ 420
Percentage of Mortgage Loans Classified as Performing	99.00%		99.00%
Financing Receivable, Modifications, Subsequent Default, Number of Contracts \| Contracts	0	0
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
Agricultural
Financing Receivable, Recorded Investment, Aging [Abstract]
Estimated fair value of mortgage loans held-for-investment \| $	$ 2,400		$ 2,200
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		$ 1,200
Financial Receivable, Recorded Investment,Past Due \| $	$ 26		26
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest \| $			$ 32

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.

+ Details

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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

+ Details

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- Details

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- Details

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- Details

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