Investments - Summary of fixed maturity and equity securities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	$ 38,932	$ 38,467
Gross Unrealized Gains	3,373	2,744
Gross Unrealized Losses	86	196
Estimated Fair Value	42,219	41,015
Corporate and other bonds
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	17,965	17,711
Gross Unrealized Gains	1,645	1,323
Gross Unrealized Losses	26	76
Estimated Fair Value	19,584	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	12,462	12,060
Gross Unrealized Gains	1,501	1,213
Gross Unrealized Losses	7	33
Estimated Fair Value	13,956	13,240
Unrealized OTTI Losses (Gains)	(14)	(16)
Residential mortgage-backed
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	4,906	5,004
Gross Unrealized Gains	127	120
Gross Unrealized Losses	28	51
Estimated Fair Value	5,005	5,073
Unrealized OTTI Losses (Gains)	(28)	(28)
Commercial mortgage-backed
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	1,858	2,016
Gross Unrealized Gains	55	48
Gross Unrealized Losses	13	24
Estimated Fair Value	1,900	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	1,047	1,022
Gross Unrealized Gains	18	8
Gross Unrealized Losses	4	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	7,811	8,042
Gross Unrealized Gains	200	176
Gross Unrealized Losses	45	80
Estimated Fair Value	7,966	8,138
Unrealized OTTI Losses (Gains)	(28)	(28)
U.S. Treasury and obligations of government-sponsored enterprises
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	115	83
Gross Unrealized Gains	3	10
Gross Unrealized Losses	3	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	439	435
Gross Unrealized Gains	10	13
Gross Unrealized Losses	4	3
Estimated Fair Value	445	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	2	1
Gross Unrealized Losses	0	0
Estimated Fair Value	20	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	38,810	38,349
Gross Unrealized Gains	3,361	2,736
Gross Unrealized Losses	85	192
Estimated Fair Value	42,086	40,893
Unrealized OTTI Losses (Gains)	(42)	(45)
Total fixed maturity securities trading
Schedule of Available-for-Sale and Trading Securities [Line Items]
Total fixed maturity securities trading, cost	4	12
Total fixed maturity securities trading, fair value	4	12
Equity securities available-for-sale -- Common stock
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	16	13
Gross Unrealized Gains	7	6
Gross Unrealized Losses	1	0
Estimated Fair Value	22	19
Preferred stock
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	102	93
Gross Unrealized Gains	5	2
Gross Unrealized Losses	0	4
Estimated Fair Value	107	91
Equity securities
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	118	106
Gross Unrealized Gains	12	8
Gross Unrealized Losses	1	4
Estimated Fair Value	$ 129	$ 110