Investments (Unconsolidated Variable Interest Entities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Carrying Amount Asset	$ 13,217	$ 14,913
Carrying Amount Liability	14,176	15,720
Other limited partnership interests
Variable Interest Entity [Line Items]
Carrying Amount Asset	1,509	1,491
Carrying Amount Liability	2,460	2,287
Real estate joint ventures
Variable Interest Entity [Line Items]
Carrying Amount Asset	24	17
Carrying Amount Liability	27	22
Other investments
Variable Interest Entity [Line Items]
Carrying Amount Asset	47	60
Carrying Amount Liability	52	66
Structured securities
Variable Interest Entity [Line Items]
Carrying Amount Asset	11,136	12,809
Carrying Amount Liability	11,136	12,809
U.S. corporate and foreign corporate
Variable Interest Entity [Line Items]
Carrying Amount Asset	501	536
Carrying Amount Liability	$ 501	$ 536