Investments - Amortized cost, gross unrealized gains and losses and fair value for fixed income securities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available for Sale Securities
Amortized cost	$ 56,831	$ 57,134
Gross unrealized, Gains	1,635	993
Gross unrealized, Losses	(264)	(957)
Fair value	58,202	57,170
U.S. government and agencies
Schedule of Available for Sale Securities
Amortized cost	3,775	5,386
Gross unrealized, Gains	119	137
Gross unrealized, Losses	(2)	(6)
Fair value	3,892	5,517
Municipal
Schedule of Available for Sale Securities
Amortized cost	8,879	8,963
Gross unrealized, Gains	393	249
Gross unrealized, Losses	(8)	(43)
Fair value	9,264	9,169
Corporate
Schedule of Available for Sale Securities
Amortized cost	41,943	40,536
Gross unrealized, Gains	998	490
Gross unrealized, Losses	(242)	(890)
Fair value	42,699	40,136
Foreign government
Schedule of Available for Sale Securities
Amortized cost	732	739
Gross unrealized, Gains	21	13
Gross unrealized, Losses	(1)	(5)
Fair value	752	747
Asset-backed securities (“ABS”)
Schedule of Available for Sale Securities
Amortized cost	1,060	1,049
Gross unrealized, Gains	7	6
Gross unrealized, Losses	(9)	(10)
Fair value	1,058	1,045
Residential mortgage-backed securities (“RMBS”)
Schedule of Available for Sale Securities
Amortized cost	354	377
Gross unrealized, Gains	89	89
Gross unrealized, Losses	(1)	(2)
Fair value	442	464
Commercial mortgage-backed securities (“CMBS”)
Schedule of Available for Sale Securities
Amortized cost	67	63
Gross unrealized, Gains	7	8
Gross unrealized, Losses	(1)	(1)
Fair value	73	70
Redeemable preferred stock
Schedule of Available for Sale Securities
Amortized cost	21	21
Gross unrealized, Gains	1	1
Gross unrealized, Losses	0	0
Fair value	$ 22	$ 22