Investments - Amortized cost, gross unrealized gains and losses and fair value for fixed income securities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available for Sale Securities
Amortized cost	$ 57,618	$ 57,525
Gross unrealized, Gains	923	1,750
Gross unrealized, Losses	(878)	(283)
Fair value	57,663	58,992
U.S. government and agencies
Schedule of Available for Sale Securities
Amortized cost	3,142	3,580
Gross unrealized, Gains	36	56
Gross unrealized, Losses	(27)	(20)
Fair value	3,151	3,616
Municipal
Schedule of Available for Sale Securities
Amortized cost	9,316	8,053
Gross unrealized, Gains	204	311
Gross unrealized, Losses	(105)	(36)
Fair value	9,415	8,328
Corporate
Schedule of Available for Sale Securities
Amortized cost	42,828	42,996
Gross unrealized, Gains	557	1,234
Gross unrealized, Losses	(723)	(204)
Fair value	42,662	44,026
Foreign government
Schedule of Available for Sale Securities
Amortized cost	854	1,005
Gross unrealized, Gains	12	27
Gross unrealized, Losses	(12)	(11)
Fair value	854	1,021
Asset-backed securities (“ABS”)
Schedule of Available for Sale Securities
Amortized cost	979	1,266
Gross unrealized, Gains	8	13
Gross unrealized, Losses	(8)	(7)
Fair value	979	1,272
Residential mortgage-backed securities (“RMBS”)
Schedule of Available for Sale Securities
Amortized cost	404	480
Gross unrealized, Gains	98	101
Gross unrealized, Losses	(2)	(3)
Fair value	500	578
Commercial mortgage-backed securities (“CMBS”)
Schedule of Available for Sale Securities
Amortized cost	74	124
Gross unrealized, Gains	7	6
Gross unrealized, Losses	(1)	(2)
Fair value	80	128
Redeemable preferred stock
Schedule of Available for Sale Securities
Amortized cost	21	21
Gross unrealized, Gains	1	2
Gross unrealized, Losses	0	0
Fair value	$ 22	$ 23