INVESTMENTS - Available-For-Sale Investments in Debt Securities, Perpetual Securities and Equity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 74,604	$ 69,072
Gross Unrealized Gains	9,677	8,722
Gross Unrealized Losses	1,075	1,092
Fair Value	83,206	76,702
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	72,244	66,363
Gross Unrealized Gains	9,206	8,432
Gross Unrealized Losses	1,035	1,035
Fair Value	80,415	73,760
Perpetual securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,549	1,506
Gross Unrealized Gains	346	176
Gross Unrealized Losses	32	49
Fair Value	1,863	1,633
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	811	1,203
Gross Unrealized Gains	125	114
Gross Unrealized Losses	8	8
Fair Value	928	1,309
Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	37,779	32,459
Gross Unrealized Gains	5,151	4,951
Gross Unrealized Losses	532	261
Fair Value	42,398	37,149
Yen-denominated | Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	555	624
Gross Unrealized Gains	95	83
Gross Unrealized Losses	2	2
Fair Value	648	705
Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	34,465	33,904
Gross Unrealized Gains	4,055	3,481
Gross Unrealized Losses	503	774
Fair Value	38,017	36,611
Dollar-denominated | Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	256	579
Gross Unrealized Gains	30	31
Gross Unrealized Losses	6	6
Fair Value	280	604
Japan government and agencies | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	27,027	22,857
Gross Unrealized Gains	3,272	3,359
Gross Unrealized Losses	423	160
Fair Value	29,876	26,056
Municipalities | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	1,292	1,295
Municipalities | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	315	246
Gross Unrealized Gains	26	29
Gross Unrealized Losses	15	8
Fair Value	326	267
Municipalities | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	818	894
Gross Unrealized Gains	148	142
Gross Unrealized Losses	0	8
Fair Value	966	1,028
Mortgage- and asset-backed securities | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	456	1,337
Mortgage- and asset-backed securities | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	248	1,096
Gross Unrealized Gains	29	33
Gross Unrealized Losses	0	8
Fair Value	277	1,121
Mortgage- and asset-backed securities | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	166	196
Gross Unrealized Gains	13	20
Gross Unrealized Losses	0	0
Fair Value	179	216
Public utilities | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	8,015	7,683
Public utilities | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,643	1,533
Gross Unrealized Gains	369	318
Gross Unrealized Losses	7	3
Fair Value	2,005	1,848
Public utilities | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	5,231	5,205
Gross Unrealized Gains	813	690
Gross Unrealized Losses	34	60
Fair Value	6,010	5,835
Sovereign and supranational | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	2,029	1,469
Sovereign and supranational | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,428	862
Gross Unrealized Gains	190	186
Gross Unrealized Losses	3	5
Fair Value	1,615	1,043
Sovereign and supranational | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	325	335
Gross Unrealized Gains	89	91
Gross Unrealized Losses	0	0
Fair Value	414	426
Banks/financial institutions | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	7,010	6,063
Banks/financial institutions | Perpetual securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	1,627	1,420
Banks/financial institutions | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,307	2,673
Gross Unrealized Gains	494	403
Gross Unrealized Losses	59	74
Fair Value	3,742	3,002
Banks/financial institutions | Yen-denominated | Perpetual securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,308	1,266
Gross Unrealized Gains	277	128
Gross Unrealized Losses	32	49
Fair Value	1,553	1,345
Banks/financial institutions | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,683	2,570
Gross Unrealized Gains	592	507
Gross Unrealized Losses	7	16
Fair Value	3,268	3,061
Banks/financial institutions | Dollar-denominated | Perpetual securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	46	51
Gross Unrealized Gains	28	24
Gross Unrealized Losses	0	0
Fair Value	74	75
Other corporate | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	31,575	29,699
Other corporate | Perpetual securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	236	213
Other corporate | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,811	3,192
Gross Unrealized Gains	771	623
Gross Unrealized Losses	25	3
Fair Value	4,557	3,812
Other corporate | Yen-denominated | Perpetual securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	195	189
Gross Unrealized Gains	41	24
Gross Unrealized Losses	0	0
Fair Value	236	213
Other corporate | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	25,092	24,556
Gross Unrealized Gains	2,388	2,021
Gross Unrealized Losses	462	690
Fair Value	27,018	25,887
U.S. government and agencies | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	150	148
Gross Unrealized Gains	12	10
Gross Unrealized Losses	0	0
Fair Value	$ 162	$ 158