INVESTMENTS - Available-for-Sale Debt Securities (Details) - USD ($) $ in Millions		Dec. 31, 2018		Dec. 31, 2017
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		$ 76,856	[1],[2]	$ 75,132	[3]
Gross Unrealized Gains		7,733		9,985
Gross Unrealized Losses		1,694		804
Fair Value	[4],[5]	82,895	[1],[2]	84,313	[3]
Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		44,027		40,378
Gross Unrealized Gains		5,308		5,582
Gross Unrealized Losses		435		350
Fair Value		48,900		45,610
Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		32,829		34,754
Gross Unrealized Gains		2,425		4,403
Gross Unrealized Losses		1,259		454
Fair Value		33,995		38,703
Japan government and agencies | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		30,637		27,980
Gross Unrealized Gains		3,700		3,363
Gross Unrealized Losses		140		271
Fair Value		34,197		31,072
U.S. government and agencies | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		137		146
Gross Unrealized Gains		9		13
Gross Unrealized Losses		1		1
Fair Value		145		158
Municipalities
Debt Securities, Available-for-Sale [Line Items]
Fair Value		1,863		1,370
Municipalities | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		385		314
Gross Unrealized Gains		32		28
Gross Unrealized Losses		9		12
Fair Value		408		330
Municipalities | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		1,343		872
Gross Unrealized Gains		120		168
Gross Unrealized Losses		8		0
Fair Value		1,455		1,040
Mortgage- and asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Fair Value		339		444
Mortgage- and asset-backed securities | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		155		242
Gross Unrealized Gains		22		29
Gross Unrealized Losses		0		0
Fair Value		177		271
Mortgage- and asset-backed securities | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		155		161
Gross Unrealized Gains		8		12
Gross Unrealized Losses		1		0
Fair Value		162		173
Public utilities
Debt Securities, Available-for-Sale [Line Items]
Fair Value		7,171		7,954
Public utilities | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		1,732		1,635
Gross Unrealized Gains		280		352
Gross Unrealized Losses		4		6
Fair Value		2,008		1,981
Public utilities | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		4,772		5,116
Gross Unrealized Gains		496		884
Gross Unrealized Losses		105		27
Fair Value		5,163		5,973
Sovereign and supranational
Debt Securities, Available-for-Sale [Line Items]
Fair Value		1,260		1,909
Sovereign and supranational | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		826		1,380
Gross Unrealized Gains		123		190
Gross Unrealized Losses		0		1
Fair Value		949		1,569
Sovereign and supranational | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		251		267
Gross Unrealized Gains		60		73
Gross Unrealized Losses		0		0
Fair Value		311		340
Banks/financial institutions
Debt Securities, Available-for-Sale [Line Items]
Fair Value		8,918		8,933
Banks/financial institutions | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		5,440		4,742
Gross Unrealized Gains		502		811
Gross Unrealized Losses		238		53
Fair Value		5,704		5,500
Banks/financial institutions | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		2,860		2,808
Gross Unrealized Gains		389		633
Gross Unrealized Losses		35		8
Fair Value		3,214		3,433
Other corporate
Debt Securities, Available-for-Sale [Line Items]
Fair Value		29,002		32,473
Other corporate | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		4,852		4,085
Gross Unrealized Gains		649		809
Gross Unrealized Losses		44		7
Fair Value		5,457		4,887
Other corporate | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost		23,311		25,384
Gross Unrealized Gains		1,343		2,620
Gross Unrealized Losses		1,109		418
Fair Value		23,545		27,586
Fixed maturity securities
Debt Securities, Available-for-Sale [Line Items]
Perpetual securities, amortized cost		1,139		1,462
Perpetual securities, fair value		$ 1,140		$ 1,789

[1] Includes perpetual securities ($1,139 at amortized cost and $1,140 at fair value)

[2] Includes perpetual securities, categorized in accordance with their respective economic maturities (the expected maturity date created by the combination of features in the financial instrument)

[3] Includes perpetual securities ($1,462 at amortized cost and $1,789 at fair value)

[4] Includes perpetual securities

[5] Includes perpetual securities