INVESTMENTS - Held-to-Maturity Debt Securities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	$ 31,259	$ 31,430
Gross Unrealized Gains	6,708	6,651
Gross Unrealized Losses	11	9
Fair Value	37,956	38,072
Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	37,956	38,072
Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	31,259	31,430
Gross Unrealized Gains	6,708	6,651
Gross Unrealized Losses	11	9
Fair Value	37,956	38,072
Japan government and agencies \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	21,808	21,331
Gross Unrealized Gains	5,409	5,160
Gross Unrealized Losses	0	0
Fair Value	27,217	26,491
Municipalities \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	363	357
Fair Value	474	462
Municipalities \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	363	357
Gross Unrealized Gains	111	105
Gross Unrealized Losses	0	0
Fair Value	474	462
Mortgage- and asset-backed securities \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	16	26
Fair Value	17	27
Mortgage- and asset-backed securities \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	16	26
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Fair Value	17	27
Public utilities \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	3,236	3,300
Fair Value	3,552	3,698
Public utilities \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	3,236	3,300
Gross Unrealized Gains	317	398
Gross Unrealized Losses	1	0
Fair Value	3,552	3,698
Sovereign and supranational \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	1,557	1,523
Fair Value	1,843	1,835
Sovereign and supranational \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	1,557	1,523
Gross Unrealized Gains	286	312
Gross Unrealized Losses	0	0
Fair Value	1,843	1,835
Banks/financial institutions \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	1,532	2,206
Fair Value	1,700	2,387
Banks/financial institutions \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	1,532	2,206
Gross Unrealized Gains	178	190
Gross Unrealized Losses	10	9
Fair Value	1,700	2,387
Other corporate \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,747	2,687
Fair Value	3,153	3,172
Other corporate \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,747	2,687
Gross Unrealized Gains	406	485
Gross Unrealized Losses	0	0
Fair Value	$ 3,153	$ 3,172

Amount of investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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