Derivatives (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Derivative assets	$ 2,244	$ 3,603
Derivative liabilities	3,129	3,883
Maximum Amount of Future Payments under Credit Default Swaps	1,878	1,891
Estimated Fair Value of Credit Default Swaps	40	28
Cash collateral on derivative assets	93	3
Cash collateral on derivative liabilities	5	25
Derivative Instrument Detail [Abstract]
Net amounts reclassified into net derivatives gains (losses) on discontinued cash flow hedges	$ 9	$ 1	$ 3
Hedging exposure to variability in future cash flows for specific length of time	2 years	3 years
Accumulated Other Comprehensive Income (Loss)	$ 231	$ 397
Customer Securities for which Entity has Right to Sell or Repledge, Fair Value of Securities Sold or Repledged	0
Over the Counter [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Excess securities collateral received on derivatives	(337)	(135)
Excess securities collateral provided on derivatives	471	108
Exchange-cleared [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Excess securities collateral provided on derivatives	426	630
Exchange Traded [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Excess securities collateral provided on derivatives	118	453
Nonperformance Risk [Member] | Direct And Assumed Guaranteed Minimum Benefit [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Embedded derivatives gains (losses)	337	200	$ 0
Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	47	(9)
Derivative liabilities	$ 104	$ (49)