Derivatives (Changes in Estimated Fair Value Related to Embedded Derivatives) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Net derivatives gains (losses)	$ 151	$ 91	$ 188	$ 248
Net derivative gains (losses)
Derivative Instruments, Gain (Loss) [Line Items]
Net derivatives gains (losses)	$ 151	$ 91	$ 188	$ 248