INVESTMENTS - Alternative Investment Funds (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Investments in Alternative Investment Funds
Alternative investment funds, fair value	$ 433	$ 404
Alternative investment funds, unfunded commitments	81	82
Hedge funds
Investments in Alternative Investment Funds
Alternative investment funds, fair value	0	1
Alternative investment funds, unfunded commitments	$ 0	$ 0
Alternative investment funds, redemption frequency (if currently eligible)	Generally quarterly
Hedge funds \| Minimum
Investments in Alternative Investment Funds
Alternative investment funds, redemption notice period	10 days	10 days
Hedge funds \| Maximum
Investments in Alternative Investment Funds
Alternative investment funds, redemption notice period	95 days	95 days
Private equity
Investments in Alternative Investment Funds
Alternative investment funds, fair value	$ 413	$ 372
Alternative investment funds, unfunded commitments	62	62
Real estate funds
Investments in Alternative Investment Funds
Alternative investment funds, fair value	20	31
Alternative investment funds, unfunded commitments	$ 19	$ 20

Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Notice Period

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- Definition

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.

+ 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 (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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- 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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- References

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