Investments (Unconsolidated Variable Interest Entities) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Carrying Amount Asset	$ 13,148	$ 13,217
Carrying Amount Liability	14,374	14,176
Other limited partnership interests
Variable Interest Entity [Line Items]
Carrying Amount Asset	1,549	1,509
Carrying Amount Liability	2,772	2,460
Other investments
Variable Interest Entity [Line Items]
Carrying Amount Asset	78	71
Carrying Amount Liability	81	79
Structured Securities
Variable Interest Entity [Line Items]
Carrying Amount Asset	11,084	11,136
Carrying Amount Liability	11,084	11,136
U.S. and foreign corporate
Variable Interest Entity [Line Items]
Carrying Amount Asset	437	501
Carrying Amount Liability	$ 437	$ 501