Reinsurance - Insurance Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums Earned
Direct	$ 14,204	$ 14,031	$ 13,811
Assumed	678	368	395
Ceded	(650)	(627)	(654)
Net earned premiums, fees and other considerations	15,121	14,554	14,361
Property and Casualty Insurance Products
Premiums Written
Direct	10,865	10,906	10,861
Assumed	223	253	297
Ceded	(571)	(591)	(580)
Net	10,517	10,568	10,578
Premiums Earned
Direct	10,923	10,871	10,704
Assumed	232	261	298
Ceded	(600)	(583)	(586)
Net	10,555	10,549	10,416
Group Benefits
Premiums Earned
Gross earned premiums, fees and other considerations	3,281	3,160	3,107
Assumed	446	107	97
Ceded	(50)	(44)	(68)
Net earned premiums, fees and other considerations	$ 3,677	$ 3,223	$ 3,136