Derivatives (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 2,045		$ 2,045		$ 2,244
Derivative Liability, Fair Value, Gross Liability	3,223		3,223		3,129
Maximum Amount of Future Payments under Credit Default Swaps	1,927		1,927		1,878
Cash collateral on derivative assets	191		191		$ 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	$ 0	$ 9
Derivative Instrument Detail [Abstract]
Hedging exposure to variability in future cash flows for specific length of time			1 year		2 years
Accumulated Other Comprehensive Income (Loss)	247		$ 247		$ 231
Cash collateral on derivative liabilities	0		0		5
Securities collateral received which the company is permitted to sell or repledge, amount that has been sold or repledged	0		0
Over the Counter [Member]
Derivatives, Fair Value [Line Items]
Cash collateral on derivative assets	(215)		(215)		(247)
Excess securities collateral received on derivatives	170		170		337
Excess securities collateral provided on derivatives	328		328		471
Exchange Cleared [Member]
Derivatives, Fair Value [Line Items]
Cash collateral on derivative assets	(16)		(16)		(27)
Excess securities collateral provided on derivatives	148		148		426
Exchange Traded [Member]
Derivatives, Fair Value [Line Items]
Excess securities collateral provided on derivatives	83		83		118
Direct And Assumed Guaranteed Minimum Benefit [Member] | Nonperformance Risk [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Embedded derivatives gains (losses)	58	$ (36)	43	$ (80)
Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	44		44		47
Derivative Liability, Fair Value, Gross Liability	$ (6)		$ (6)		$ (9)