INVESTMENTS - Held-to-Maturity Investments in Debt Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	$ 31,430	$ 33,350
Gross Unrealized Gains	6,651	6,735
Gross Unrealized Losses	9	64
Fair Value	38,072	40,021
Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	38,072	40,021
Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	31,430	33,350
Gross Unrealized Gains	6,651	6,735
Gross Unrealized Losses	9	64
Fair Value	38,072	40,021
Japan government and agencies \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	21,331
Gross Unrealized Gains	5,160	5,338
Gross Unrealized Losses	0	0
Fair Value	26,491	26,040
Municipalities \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	357	350
Fair Value	462	457
Municipalities \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	357
Gross Unrealized Gains	105	107
Gross Unrealized Losses	0	0
Fair Value	462	457
Mortgage- and asset-backed securities \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	26	30
Fair Value	27	32
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	26
Gross Unrealized Gains	1	2
Gross Unrealized Losses	0	0
Fair Value	27	32
Public utilities \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	3,300	3,201
Fair Value	3,698	3,536
Public utilities \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	3,300
Gross Unrealized Gains	398	358
Gross Unrealized Losses	0	23
Fair Value	3,698	3,536
Sovereign and supranational \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	1,523	2,602
Fair Value	1,835	2,877
Sovereign and supranational \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	1,523
Gross Unrealized Gains	312	283
Gross Unrealized Losses	0	8
Fair Value	1,835	2,877
Banks/financial institutions \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,206	3,731
Fair Value	2,387	3,900
Banks/financial institutions \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,206
Gross Unrealized Gains	190	195
Gross Unrealized Losses	9	26
Fair Value	2,387	3,900
Other corporate \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,687	2,734
Fair Value	3,172	3,179
Other corporate \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,687
Gross Unrealized Gains	485	452
Gross Unrealized Losses	0	7
Fair Value	$ 3,172	$ 3,179

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 4: 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)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.6)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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