Reinsurance - Insurance Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Premiums Earned
Direct	$ 16,132	$ 14,439	$ 14,204
Assumed	1,988	2,265	678
Ceded	(1,027)	(660)	(650)
Premiums Earned, Net	16,923	15,869	14,141
Property and Casualty Insurance Products
Premiums Written
Direct	12,190	10,784	10,865
Assumed	371	217	223
Ceded	(978)	(593)	(571)
Net	11,583	10,408	10,517
Premiums Earned
Direct	12,010	10,824	10,923
Assumed	416	221	232
Ceded	(936)	(599)	(600)
Net	11,490	10,446	10,555
Premiums Earned, Net	11,490	10,446	10,555
Group Benefits [Member]
Premiums Earned
Direct	4,122	3,615	3,281
Assumed	1,572	2,044	446
Ceded	(91)	(61)	(50)
Premiums Earned, Net	$ 5,603	$ 5,598	$ 3,677