Derivatives (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 2,194		$ 2,244
Derivative Liability, Fair Value, Gross Liability	3,141		3,129
Maximum Amount of Future Payments under Credit Default Swaps	1,906		1,878
Cash collateral on derivative assets	180		$ 93
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring, Net	$ 0	$ 9
Derivative Instrument Detail [Abstract]
Hedging exposure to variability in future cash flows for specific length of time	2 years		2 years
Accumulated Other Comprehensive Income (Loss)	$ 150		$ 231
Cash collateral on derivative liabilities	0		5
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]
Cash collateral on derivative assets	(262)		(247)
Excess securities collateral received on derivatives	292		337
Excess securities collateral provided on derivatives	595		471
Exchange Cleared [Member]
Derivatives, Fair Value [Line Items]
Cash collateral on derivative assets	(19)		(27)
Excess securities collateral provided on derivatives	145		426
Exchange Traded [Member]
Derivatives, Fair Value [Line Items]
Excess securities collateral provided on derivatives	98		118
Direct And Assumed Guaranteed Minimum Benefit [Member] | Nonperformance Risk [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Embedded derivatives gains (losses)	(15)	$ (44)
Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	68		47
Derivative Liability, Fair Value, Gross Liability	$ 1		$ (9)