Investments - Credit Risk Profile, Commercial Mortgage Loans (Details)	12 Months Ended
Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Commercial mortgage loan	$ 1,761,000,000	$ 1,666,000,000
Weighted Average [Member]
Financing Receivable, Recorded Investment [Line Items]
Average Debt Service Coverage Ratio	2.11	1.95
Average Loan-to-Value Ratio	57.00%	57.00%
Below 60% [Member]
Financing Receivable, Recorded Investment [Line Items]
Commercial mortgage loan	$ 1,109,000,000	$ 943,000,000
Below 60% [Member] \| Weighted Average [Member]
Financing Receivable, Recorded Investment [Line Items]
Average Debt Service Coverage Ratio	2.03	2.06
60% to 79% [Member]
Financing Receivable, Recorded Investment [Line Items]
Commercial mortgage loan	$ 652,000,000	$ 702,000,000
60% to 79% [Member] \| Weighted Average [Member]
Financing Receivable, Recorded Investment [Line Items]
Average Debt Service Coverage Ratio	2.24	1.89
80% to 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Commercial mortgage loan	$ 0	$ 21,000,000
80% to 100% [Member] \| Weighted Average [Member]
Financing Receivable, Recorded Investment [Line Items]
Average Debt Service Coverage Ratio	0	0

Represents ratio measuring the amount of property cash flow available for annual interest and principal payments on debt.

No definition available.

+ Details

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- Definition

Represents the percentage of the amount of loan to the fair value of the underlying property collateralizing the loan.

+ References

No definition available.

+ Details

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- Definition

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.

+ References

No definition available.

+ Details

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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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