Fair Value Measurements - Separate Account Assets Priced at NAV (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 65
Minimum [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period	45 days
Maximum [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period	90 days
Fair Value, Measurements, Recurring [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Frequency	Quarterly redemption frequency
Separate Account Assets [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 302
Separate Account Assets [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Fair Value	776	$ 774
Security Partnerships [Member] \| Separate Account Assets [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 302
Redemption Frequency	Not applicable
Security Partnerships [Member] \| Separate Account Assets [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 485	$ 458
Real Estate Funds [Member] \| Separate Account Assets [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 0
Redemption Frequency	Quarterly
Real Estate Funds [Member] \| Separate Account Assets [Member] \| Minimum [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period	45 days	45 days
Real Estate Funds [Member] \| Separate Account Assets [Member] \| Maximum [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period	90 days	90 days
Real Estate Funds [Member] \| Separate Account Assets [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 247	$ 239
Hedge Funds [Member] \| Separate Account Assets [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 0
Redemption Frequency	Up to Annually, varying by fund
Hedge Funds [Member] \| Separate Account Assets [Member] \| Minimum [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period	30 days	30 days
Hedge Funds [Member] \| Separate Account Assets [Member] \| Maximum [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period	90 days	90 days
Hedge Funds [Member] \| Separate Account Assets [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 44	$ 77

Fair value of investment in certain entities that calculate net asset value per share. Includes, but is not limited to, by unit, membership interest, or other ownership interest. Investment includes, but is not limited to, investment in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds. Excludes measurement of net asset value as estimate of fair value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

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- Definition

States the frequency (for example, annually, semi-annually, quarterly) with which the entity can redeem its investment, or portion thereof, for each of those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments), by major category.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

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Notice period the entity is required to deliver before it can redeem an investment calculated using net asset value per share, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, investments calculated by per unit, per membership interest, other equity or ownership interest and alternative investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

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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.

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- Definition

The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

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