Investments - Summary of fixed maturity and equity securities (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	$ 38,381	$ 38,467
Gross Unrealized Gains	2,863	2,744
Gross Unrealized Losses	144	196
Estimated Fair Value	41,100	41,015
Corporate and other bonds
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	17,838	17,711
Gross Unrealized Gains	1,428	1,323
Gross Unrealized Losses	37	76
Estimated Fair Value	19,229	18,958
Unrealized OTTI Losses (Gains)	(1)	(1)
States, municipalities and political subdivisions
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	12,261	12,060
Gross Unrealized Gains	1,219	1,213
Gross Unrealized Losses	31	33
Estimated Fair Value	13,449	13,240
Unrealized OTTI Losses (Gains)	(12)	(16)
Residential mortgage-backed
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	4,672	5,004
Gross Unrealized Gains	121	120
Gross Unrealized Losses	50	51
Estimated Fair Value	4,743	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	1,902	2,016
Gross Unrealized Gains	52	48
Gross Unrealized Losses	19	24
Estimated Fair Value	1,935	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	1,043	1,022
Gross Unrealized Gains	10	8
Gross Unrealized Losses	4	5
Estimated Fair Value	1,049	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	7,617	8,042
Gross Unrealized Gains	183	176
Gross Unrealized Losses	73	80
Estimated Fair Value	7,727	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	95	83
Gross Unrealized Gains	8	10
Gross Unrealized Losses	0	0
Estimated Fair Value	103	93
Unrealized OTTI Losses (Gains)	0	0
Foreign government
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	419	435
Gross Unrealized Gains	14	13
Gross Unrealized Losses	1	3
Estimated Fair Value	432	445
Unrealized OTTI Losses (Gains)	0	0
Redeemable preferred stock
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	19	18
Gross Unrealized Gains	1	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	38,249	38,349
Gross Unrealized Gains	2,853	2,736
Gross Unrealized Losses	142	192
Estimated Fair Value	40,960	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	20	12
Total fixed maturity securities trading, fair value	20	12
Equity securities available-for-sale -- Common stock
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	20	13
Gross Unrealized Gains	5	6
Gross Unrealized Losses	0	0
Estimated Fair Value	25	19
Preferred stock
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	92	93
Gross Unrealized Gains	5	2
Gross Unrealized Losses	2	4
Estimated Fair Value	95	91
Equity securities
Schedule of Available-for-Sale and Trading Securities [Line Items]
Cost or Amortized Cost	112	106
Gross Unrealized Gains	10	8
Gross Unrealized Losses	2	4
Estimated Fair Value	$ 120	$ 110