INVESTMENTS - Alternative Investment Funds (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Investments in Alternative Investment Funds
Alternative investment funds, unfunded commitments	$ 81	$ 81
Hedge funds
Investments in Alternative Investment Funds
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, unfunded commitments	$ 62	$ 62
Real estate funds
Investments in Alternative Investment Funds
Alternative investment funds, unfunded commitments	19	19
Mutual/collective investment funds
Investments in Alternative Investment Funds
Alternative investment funds, unfunded commitments	0	0
Fair value
Investments in Alternative Investment Funds
Alternative investment funds, fair value	199	207
Fair value \| Hedge funds
Investments in Alternative Investment Funds
Alternative investment funds, fair value	0	0
Fair value \| Private equity
Investments in Alternative Investment Funds
Alternative investment funds, fair value	160	168
Fair value \| Real estate funds
Investments in Alternative Investment Funds
Alternative investment funds, fair value	14	14
Fair value \| Mutual/collective investment funds
Investments in Alternative Investment Funds
Alternative investment funds, fair value	$ 25	$ 25

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

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Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 35
-Paragraph 54B
-URI http://asc.fasb.org/extlink&oid=99392525&loc=SL7495116-110257

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

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