Derivatives (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Maximum Amount of Future Payments under Credit Default Swaps	$ 11,353		$ 11,520
Credit Risk Derivative Assets, at Fair Value	(60)
Estimated Fair Value of Credit Default Swaps			271
Estimated Fair Value Assets	18,869		10,084
Estimated Fair Value Liabilities	4,849		3,737
Excess securities collateral received on derivatives	285		$ 389
Collateral Amount Not Provided Due to Downgrade Threshold	15
Derivative Instrument Detail [Abstract]
Net amounts reclassified into net derivatives gains (losses) on discontinued cash flow hedges	$ (5)	$ 1
Hedging exposure to variability in future cash flows for specific length of time	9 years		8 years
Accumulated Other Comprehensive Income Loss	$ 6,300		$ 2,200
Cumulative foreign currency translation gain (loss) recorded in accumulated other comprehensive income (loss) for net investment in foreign operations hedges	256		148
Excess securities collateral provided on derivatives	456		266
Securities collateral received which the company is permitted to sell or repledge, amount that has been sold or repledged	0
Nonperformance Risk [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net derivatives gains (losses)	185	$ (62)
Over the Counter [Member]
Derivatives, Fair Value [Line Items]
Excess securities collateral received on derivatives	(336)		(156)
Excess securities collateral provided on derivatives	(355)		(189)
Cash collateral on derivative assets	(11,556)		(5,317)
Exchange-traded
Derivatives, Fair Value [Line Items]
Excess securities collateral received on derivatives	(2,200)		(1,000)
Excess securities collateral provided on derivatives	(158)		(143)
Cash collateral on derivative assets	0		0
Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Estimated Fair Value Assets	162		111
Estimated Fair Value Liabilities	$ 34		$ (21)