Derivatives (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Sep. 30, 2015	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Maximum Amount of Future Payments under Credit Default Swaps	$ 12,369			$ 12,645
Estimated Fair Value of Credit Default Swaps	199			180
Estimated Fair Value Assets	12,278			15,761
Estimated Fair Value Liabilities	8,399			10,804
Excess securities collateral received on derivatives	232			$ 168
Derivative Instruments, Gain (Loss) [Line Items]
Net derivatives gains (losses)	578	$ (1,169)
Derivative Instrument Detail [Abstract]
Change in fair value of derivatives excluded from the assessment of hedge effectiveness	(7)	(6)
Net amounts reclassified into net derivatives gains (losses) on discontinued cash flow hedges	$ 32		$ (4)
Hedging exposure to variability in future cash flows for specific length of time	5 years			5 years
Accumulated Other Comprehensive Income Loss	$ 2,900			$ 2,900
Deferred net gains (losses) expected to be reclassified to earnings	6
Amount and Location of Gains (Losses) Reclassified from Accumulated Other Comprehensive Income (Loss) into Income (Loss) (Effective Portion)	$ 0	$ 0
Immateriality of cash flow effectiveness	0	0
Cumulative foreign currency translation gain (loss) recorded in accumulated other comprehensive income (loss) for net investment in foreign operations hedges	$ 428			754
Potential future recoveries available to offset maximum amount of future payments under credit default swaps	217			30
Excess securities collateral provided on derivatives	366			486
Securities collateral received which the company is permitted to sell or repledge, amount that has been sold or repledged	0
Over the Counter [Member]
Derivatives, Fair Value [Line Items]
Excess securities collateral received on derivatives	(221)			(217)
Excess securities collateral provided on derivatives	(158)			(297)
Cash collateral on derivative assets	(4,302)			(4,897)
Exchange-traded
Derivatives, Fair Value [Line Items]
Excess securities collateral received on derivatives	(732)			(1,200)
Excess securities collateral provided on derivatives	(325)			(569)
Cash collateral on derivative assets	0			0
Nonperformance Risk [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net derivatives gains (losses)	(43)	$ 359
Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Estimated Fair Value Assets	193			249
Estimated Fair Value Liabilities	$ (64)			$ (92)