Investments (Mortgage Loans and Valuation Allowance by Portfolio Segment) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	$ 22	$ 11
Recorded Investment	21	10
Valuation Allowances	2	1
Unpaid Principal Balance	385	304
Recorded Investment	351	280
Recorded Investment	68,153	64,615
Valuation Allowances	312	303
Carrying Value	370	289
Average Recorded Investment	322	327
Commercial
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	0	0
Recorded Investment	0	0
Valuation Allowances	0	0
Unpaid Principal Balance	0	12
Recorded Investment	0	12
Recorded Investment	44,375	41,500
Valuation Allowances	214	202
Carrying Value	0	12
Average Recorded Investment	5	90	$ 127
Agricultural
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	22	11
Recorded Investment	21	10
Valuation Allowances	2	1
Unpaid Principal Balance	27	27
Recorded Investment	27	27
Recorded Investment	12,966	12,527
Valuation Allowances	39	38
Carrying Value	46	36
Average Recorded Investment	32	49	60
Residential
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	0	0
Recorded Investment	0	0
Valuation Allowances	0	0
Unpaid Principal Balance	358	265
Recorded Investment	324	241
Recorded Investment	10,812	10,588
Valuation Allowances	59	63
Carrying Value	324	241
Average Recorded Investment	$ 285	$ 188	$ 84

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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

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

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+ 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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

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