INVESTMENTS - Available-For-Sale Investments in Debt Securities, Perpetual Securities and Equity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 75,978	$ 69,072
Gross Unrealized Gains	10,165	8,722
Gross Unrealized Losses	807	1,092
Fair Value	85,336	76,702
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	73,670	66,363
Gross Unrealized Gains	9,631	8,432
Gross Unrealized Losses	777	1,035
Fair Value	82,524	73,760
Perpetual securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,462	1,506
Gross Unrealized Gains	354	176
Gross Unrealized Losses	27	49
Fair Value	1,789	1,633
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	846	1,203
Gross Unrealized Gains	180	114
Gross Unrealized Losses	3	8
Fair Value	1,023	1,309
Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	38,962	32,459
Gross Unrealized Gains	5,257	4,951
Gross Unrealized Losses	323	261
Fair Value	43,896	37,149
Yen-denominated | Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	561	624
Gross Unrealized Gains	135	83
Gross Unrealized Losses	1	2
Fair Value	695	705
Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	34,708	33,904
Gross Unrealized Gains	4,374	3,481
Gross Unrealized Losses	454	774
Fair Value	38,628	36,611
Dollar-denominated | Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	285	579
Gross Unrealized Gains	45	31
Gross Unrealized Losses	2	6
Fair Value	328	604
Japan government and agencies | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	27,980	22,857
Gross Unrealized Gains	3,363	3,359
Gross Unrealized Losses	271	160
Fair Value	31,072	26,056
Municipalities | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	1,370	1,295
Municipalities | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	314	246
Gross Unrealized Gains	28	29
Gross Unrealized Losses	12	8
Fair Value	330	267
Municipalities | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	872	894
Gross Unrealized Gains	168	142
Gross Unrealized Losses	0	8
Fair Value	1,040	1,028
Mortgage- and asset-backed securities | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	444	1,337
Mortgage- and asset-backed securities | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	242	1,096
Gross Unrealized Gains	29	33
Gross Unrealized Losses	0	8
Fair Value	271	1,121
Mortgage- and asset-backed securities | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	161	196
Gross Unrealized Gains	12	20
Gross Unrealized Losses	0	0
Fair Value	173	216
Public utilities | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	7,954	7,683
Public utilities | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,635	1,533
Gross Unrealized Gains	352	318
Gross Unrealized Losses	6	3
Fair Value	1,981	1,848
Public utilities | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	5,116	5,205
Gross Unrealized Gains	884	690
Gross Unrealized Losses	27	60
Fair Value	5,973	5,835
Sovereign and supranational | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	1,909	1,469
Sovereign and supranational | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,380	862
Gross Unrealized Gains	190	186
Gross Unrealized Losses	1	5
Fair Value	1,569	1,043
Sovereign and supranational | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	267	335
Gross Unrealized Gains	73	91
Gross Unrealized Losses	0	0
Fair Value	340	426
Banks/financial institutions | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	7,377	6,063
Banks/financial institutions | Perpetual securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	1,556	1,420
Banks/financial institutions | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,521	2,673
Gross Unrealized Gains	524	403
Gross Unrealized Losses	26	74
Fair Value	4,019	3,002
Banks/financial institutions | Yen-denominated | Perpetual securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,221	1,266
Gross Unrealized Gains	287	128
Gross Unrealized Losses	27	49
Fair Value	1,481	1,345
Banks/financial institutions | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,762	2,570
Gross Unrealized Gains	604	507
Gross Unrealized Losses	8	16
Fair Value	3,358	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	29	24
Gross Unrealized Losses	0	0
Fair Value	75	75
Other corporate | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	32,240	29,699
Other corporate | Perpetual securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	233	213
Other corporate | Yen-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,890	3,192
Gross Unrealized Gains	771	623
Gross Unrealized Losses	7	3
Fair Value	4,654	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	38	24
Gross Unrealized Losses	0	0
Fair Value	233	213
Other corporate | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	25,384	24,556
Gross Unrealized Gains	2,620	2,021
Gross Unrealized Losses	418	690
Fair Value	27,586	25,887
U.S. Government and Agencies | Dollar-denominated | Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	146	148
Gross Unrealized Gains	13	10
Gross Unrealized Losses	1	0
Fair Value	$ 158	$ 158