INVESTMENTS - Held-to-Maturity Investments in Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	$ 33,377	$ 31,430
Gross Unrealized Gains	7,104	6,651
Gross Unrealized Losses	13	9
Fair Value	40,468	38,072
Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	40,468	38,072
Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	33,377	31,430
Gross Unrealized Gains	7,104	6,651
Gross Unrealized Losses	13	9
Fair Value	40,468	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	22,686	21,331
Gross Unrealized Gains	5,631	5,160
Gross Unrealized Losses	0	0
Fair Value	28,317	26,491
Municipalities \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	379	357
Fair Value	493	462
Municipalities \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	379	357
Gross Unrealized Gains	114	105
Gross Unrealized Losses	0	0
Fair Value	493	462
Mortgage- and asset-backed securities \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	26	26
Fair Value	28	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	26	26
Gross Unrealized Gains	2	1
Gross Unrealized Losses	0	0
Fair Value	28	27
Public utilities \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	3,508	3,300
Fair Value	3,891	3,698
Public utilities \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	3,508	3,300
Gross Unrealized Gains	383	398
Gross Unrealized Losses	0	0
Fair Value	3,891	3,698
Sovereign and supranational \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	1,620	1,523
Fair Value	1,919	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	1,620	1,523
Gross Unrealized Gains	299	312
Gross Unrealized Losses	0	0
Fair Value	1,919	1,835
Banks/financial institutions \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,300	2,206
Fair Value	2,477	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	2,300	2,206
Gross Unrealized Gains	190	190
Gross Unrealized Losses	13	9
Fair Value	2,477	2,387
Other corporate \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,858	2,687
Fair Value	3,343	3,172
Other corporate \| Yen-denominated \| Fixed maturities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, fixed maturities, amortized cost	2,858	2,687
Gross Unrealized Gains	485	485
Gross Unrealized Losses	0	0
Fair Value	$ 3,343	$ 3,172

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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