Investments - Carrying value of non-impaired mortgage loans summarized by debt service coverage ratio distribution (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Mortgage Loans on Real Estate, Commercial and Consumer, Net	$ 4,679	$ 4,534
Mortgage loans, non-impaired
Financing Receivable, Impaired [Line Items]
Fixed rate mortgage loans	4,545	4,438
Variable rate mortgage loans	130	92
Mortgage Loans on Real Estate, Commercial and Consumer, Net	4,675	4,530
Below 1.0 \| Mortgage loans, non-impaired
Financing Receivable, Impaired [Line Items]
Fixed rate mortgage loans	27	3
Variable rate mortgage loans	0	0
Mortgage Loans on Real Estate, Commercial and Consumer, Net	27	3
1.0 - 1.25 \| Mortgage loans, non-impaired
Financing Receivable, Impaired [Line Items]
Fixed rate mortgage loans	355	345
Variable rate mortgage loans	0	0
Mortgage Loans on Real Estate, Commercial and Consumer, Net	355	345
1.26 - 1.50 \| Mortgage loans, non-impaired
Financing Receivable, Impaired [Line Items]
Fixed rate mortgage loans	1,165	1,141
Variable rate mortgage loans	30	30
Mortgage Loans on Real Estate, Commercial and Consumer, Net	1,195	1,171
Above 1.50 \| Mortgage loans, non-impaired
Financing Receivable, Impaired [Line Items]
Fixed rate mortgage loans	2,998	2,949
Variable rate mortgage loans	100	62
Mortgage Loans on Real Estate, Commercial and Consumer, Net	$ 3,098	$ 3,011

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.

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- Definition

Amount of loan with fixed rate of interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

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- Definition

Amount of loan with variable or adjustable rate of interest.

+ References

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(c))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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