INVESTMENTS - Schedule of Available-for-sale Securities (Details) - USD ($) $ in Millions		Mar. 31, 2018		Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		$ 81,607	[1]	$ 75,132	[2]
Gross Unrealized Gains		8,998		9,985
Gross Unrealized Losses		1,572		804
Fair Value	[3]	89,033	[1]	84,313	[2]
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value				1,023
Perpetual Securities, Fair Value		71
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	[3]	89,033		84,313
Perpetual Securities, Amortized Cost		1,506		1,462
Perpetual Securities, Fair Value		1,774		1,789
Yen-denominated | Equity securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value				695
Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		46,154		40,378
Gross Unrealized Gains		5,964		5,582
Gross Unrealized Losses		318		350
Fair Value		51,800		45,610
Dollar-denominated | Equity securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value				328
Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		35,453		34,754
Gross Unrealized Gains		3,034		4,403
Gross Unrealized Losses		1,254		454
Fair Value		37,233		38,703
Japan government and agencies | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		32,961		27,980
Gross Unrealized Gains		3,811		3,363
Gross Unrealized Losses		192		271
Fair Value		36,580		31,072
Municipalities | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value		1,373		1,370
Municipalities | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		350		314
Gross Unrealized Gains		33		28
Gross Unrealized Losses		11		12
Fair Value		372		330
Municipalities | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		866		872
Gross Unrealized Gains		135		168
Gross Unrealized Losses		0		0
Fair Value		1,001		1,040
Mortgage- and asset-backed securities | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value		455		444
Mortgage- and asset-backed securities | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		254		242
Gross Unrealized Gains		30		29
Gross Unrealized Losses		0		0
Fair Value		284		271
Mortgage- and asset-backed securities | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		165		161
Gross Unrealized Gains		7		12
Gross Unrealized Losses		1		0
Fair Value		171		173
Public utilities | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value		7,807		7,954
Public utilities | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		1,739		1,635
Gross Unrealized Gains		325		352
Gross Unrealized Losses		4		6
Fair Value		2,060		1,981
Public utilities | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		5,223		5,116
Gross Unrealized Gains		631		884
Gross Unrealized Losses		107		27
Fair Value		5,747		5,973
Sovereign and supranational | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value		1,866		1,909
Sovereign and supranational | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		1,348		1,380
Gross Unrealized Gains		212		190
Gross Unrealized Losses		27		1
Fair Value		1,533		1,569
Sovereign and supranational | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		274		267
Gross Unrealized Gains		59		73
Gross Unrealized Losses		0		0
Fair Value		333		340
Banks/financial institutions | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value		9,144		8,933
Banks/financial institutions | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		5,157		4,742
Gross Unrealized Gains		799		811
Gross Unrealized Losses		76		53
Fair Value		5,880		5,500
Banks/financial institutions | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		2,841		2,808
Gross Unrealized Gains		457		633
Gross Unrealized Losses		34		8
Fair Value		3,264		3,433
Other corporate | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value		31,652		32,473
Other corporate | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		4,345		4,085
Gross Unrealized Gains		754		809
Gross Unrealized Losses		8		7
Fair Value		5,091		4,887
Other corporate | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		25,936		25,384
Gross Unrealized Gains		1,736		2,620
Gross Unrealized Losses		1,111		418
Fair Value		26,561		27,586
U.S. government and agencies | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		148		146
Gross Unrealized Gains		9		13
Gross Unrealized Losses		1		1
Fair Value		$ 156		$ 158

[1] Includes perpetual securities ($1,506 at amortized cost and $1,774 at fair value)

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

[3] Includes perpetual securities