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INVESTMENTS - Held-to-Maturity Investments in Debt Securities (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Held-to-maturity Securities [Line Items] |  |
Securities held to maturity, fixed maturities, amortized cost | $ 31,998 | $ 33,350
Gross Unrealized Gains | 6,619 | 6,735
Gross Unrealized Losses | 18 | 64
Fair Value | 38,599 | 40,021
Fixed maturities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Fair Value | 38,599 | 40,021
Yen-denominated | Fixed maturities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Securities held to maturity, fixed maturities, amortized cost | 31,998 | 33,350
Gross Unrealized Gains | 6,619 | 6,735
Gross Unrealized Losses | 18 | 64
Fair Value | 38,599 | 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 | 21,385 | 20,702
Gross Unrealized Gains | 5,070 | 5,338
Gross Unrealized Losses | 0 | 0
Fair Value | 26,455 | 26,040
Municipalities | Fixed maturities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Securities held to maturity, fixed maturities, amortized cost | 359 | 350
Fair Value | 463 | 457
Municipalities | Yen-denominated | Fixed maturities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Securities held to maturity, fixed maturities, amortized cost | 359 | 350
Gross Unrealized Gains | 104 | 107
Gross Unrealized Losses | 0 | 0
Fair Value | 463 | 457
Mortgage- and asset-backed securities | Fixed maturities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Securities held to maturity, fixed maturities, amortized cost | 27 | 30
Fair Value | 29 | 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 | 27 | 30
Gross Unrealized Gains | 2 | 2
Gross Unrealized Losses | 0 | 0
Fair Value | 29 | 32
Public utilities | Fixed maturities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Securities held to maturity, fixed maturities, amortized cost | 3,308 | 3,201
Fair Value | 3,734 | 3,536
Public utilities | Yen-denominated | Fixed maturities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Securities held to maturity, fixed maturities, amortized cost | 3,308 | 3,201
Gross Unrealized Gains | 426 | 358
Gross Unrealized Losses | 0 | 23
Fair Value | 3,734 | 3,536
Sovereign and supranational | Fixed maturities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Securities held to maturity, fixed maturities, amortized cost | 1,527 | 2,602
Fair Value | 1,846 | 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 | 1,527 | 2,602
Gross Unrealized Gains | 319 | 283
Gross Unrealized Losses | 0 | 8
Fair Value | 1,846 | 2,877
Banks/financial institutions | Fixed maturities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Securities held to maturity, fixed maturities, amortized cost | 2,700 | 3,731
Fair Value | 2,887 | 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 | 2,700 | 3,731
Gross Unrealized Gains | 205 | 195
Gross Unrealized Losses | 18 | 26
Fair Value | 2,887 | 3,900
Other corporate | Fixed maturities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Securities held to maturity, fixed maturities, amortized cost | 2,692 | 2,734
Fair Value | 3,185 | 3,179
Other corporate | Yen-denominated | Fixed maturities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Securities held to maturity, fixed maturities, amortized cost | 2,692 | 2,734
Gross Unrealized Gains | 493 | 452
Gross Unrealized Losses | 0 | 7
Fair Value | $ 3,185 | $ 3,179

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