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A valuation allowance for financing receivables that are expected to be uncollectible.

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

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No definition available.

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Amount of direct write-downs of financing receivables charged against the allowance.

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
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No definition available.

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

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 14A
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No definition available.

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Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

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Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

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Amount of loan with fixed rate of interest.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
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-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

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Amount of loan with variable or adjustable rate of interest.

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The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Investments - Nonimpaired and impaired mortgage loans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Carrying value of non-impaired mortgage loans, summarized by debt service coverage ratio distribution
Total				$ 4,534	$ 4,486
Mortgage loans, non-impaired
Carrying value of non-impaired mortgage loans, summarized by debt service coverage ratio distribution
Fixed rate mortgage loans				4,438	4,442
Variable rate mortgage loans				92	39
Total				4,530	4,481
Net carrying value of impaired mortgage loans
Impaired mortgage loans with a valuation allowance				4	5
Impaired mortgage loans without a valuation allowance				0	0
Total impaired mortgage loans				4	5
Valuation allowance on impaired mortgage loans	$ 3	$ 3	$ 8	3	3
Rollforward of the valuation allowance on impaired mortgage loans
Beginning balance	3	3	8
Net increase (decrease) in valuation allowance	1	0	(4)
Charge offs	(1)	0	(1)
Ending balance	3	3	3
Mortgage loans, non-impaired \| Below 1.0
Carrying value of non-impaired mortgage loans, summarized by debt service coverage ratio distribution
Fixed rate mortgage loans				3	60
Variable rate mortgage loans				0	0
Total				3	60
Mortgage loans, non-impaired \| 1.0 - 1.25
Carrying value of non-impaired mortgage loans, summarized by debt service coverage ratio distribution
Fixed rate mortgage loans				345	324
Variable rate mortgage loans				0	0
Total				345	324
Mortgage loans, non-impaired \| 1.26 - 1.50
Carrying value of non-impaired mortgage loans, summarized by debt service coverage ratio distribution
Fixed rate mortgage loans				1,141	1,293
Variable rate mortgage loans				30	0
Total				1,171	1,293
Mortgage loans, non-impaired \| Above 1.50
Carrying value of non-impaired mortgage loans, summarized by debt service coverage ratio distribution
Fixed rate mortgage loans				2,949	2,765
Variable rate mortgage loans				62	39
Total				$ 3,011	$ 2,804
Mortgage loans
Average carrying value and interest income recognized on impaired mortgage loans
Average impaired mortgage loans	$ 7	$ 6	$ 11