Investments - Summary of fixed maturity and equity securities (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	$ 38,776	$ 38,467
Gross Unrealized Gains	3,196	2,744
Gross Unrealized Losses	105	196
Estimated Fair Value	41,867	41,015
Corporate and other bonds
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	17,823	17,711
Gross Unrealized Gains	1,589	1,323
Gross Unrealized Losses	29	76
Estimated Fair Value	19,383	18,958
Unrealized OTTI Losses (Gains)	0	(1)
States, municipalities and political subdivisions
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	12,461	12,060
Gross Unrealized Gains	1,380	1,213
Gross Unrealized Losses	15	33
Estimated Fair Value	13,826	13,240
Unrealized OTTI Losses (Gains)	(13)	(16)
Residential mortgage-backed
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	4,835	5,004
Gross Unrealized Gains	124	120
Gross Unrealized Losses	38	51
Estimated Fair Value	4,921	5,073
Unrealized OTTI Losses (Gains)	(27)	(28)
Commercial mortgage-backed
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	1,907	2,016
Gross Unrealized Gains	59	48
Gross Unrealized Losses	14	24
Estimated Fair Value	1,952	2,040
Unrealized OTTI Losses (Gains)	0	0
Other asset-backed
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	1,050	1,022
Gross Unrealized Gains	16	8
Gross Unrealized Losses	5	5
Estimated Fair Value	1,061	1,025
Unrealized OTTI Losses (Gains)	0	0
Total asset-backed
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	7,792	8,042
Gross Unrealized Gains	199	176
Gross Unrealized Losses	57	80
Estimated Fair Value	7,934	8,138
Unrealized OTTI Losses (Gains)	(27)	(28)
U.S. Treasury and obligations of government-sponsored enterprises
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	113	83
Gross Unrealized Gains	4	10
Gross Unrealized Losses	2	0
Estimated Fair Value	115	93
Unrealized OTTI Losses (Gains)	0	0
Foreign government
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	438	435
Gross Unrealized Gains	12	13
Gross Unrealized Losses	1	3
Estimated Fair Value	449	445
Unrealized OTTI Losses (Gains)	0	0
Redeemable preferred stock
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	18	18
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Estimated Fair Value	19	19
Unrealized OTTI Losses (Gains)	0	0
Total fixed maturity securities available-for-sale
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	38,645	38,349
Gross Unrealized Gains	3,185	2,736
Gross Unrealized Losses	104	192
Estimated Fair Value	41,726	40,893
Unrealized OTTI Losses (Gains)	(40)	(45)
Total fixed maturity securities trading
Schedule of Available-for-Sale and Trading Securities [Line Items]
Total fixed maturity securities trading, cost	23	12
Total fixed maturity securities trading, fair value	23	12
Equity securities available-for-sale -- Common stock
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	17	13
Gross Unrealized Gains	5	6
Gross Unrealized Losses	0	0
Estimated Fair Value	22	19
Preferred stock
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	91	93
Gross Unrealized Gains	6	2
Gross Unrealized Losses	1	4
Estimated Fair Value	96	91
Equity securities
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	108	106
Gross Unrealized Gains	11	8
Gross Unrealized Losses	1	4
Estimated Fair Value	$ 118	$ 110