Derivative Financial Instruments - Schedule of Outstanding Hedges: (Notional Amount) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		$ 10,994	$ 11,047
Stable value protection		26,313	26,653
Derivatives not designated as hedging instruments | Interest-rate contracts | Futures
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		14,656	2,392
Derivatives not designated as hedging instruments | Interest-rate contracts | Swap agreements
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		300	0
Derivatives not designated as hedging instruments | Foreign exchange contracts | Futures
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		0	50
Derivatives not designated as hedging instruments | Foreign exchange contracts | Forward, swap and spot
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		2,389,839	1,679,976
Derivatives not designated as hedging instruments | Foreign exchange contracts | Options purchased
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		452	350
Derivatives not designated as hedging instruments | Foreign exchange contracts | Options written
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		267	302
Derivatives not designated as hedging instruments | Commodity and Equity Contracts | Commodity
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		73	16
Derivatives not designated as hedging instruments | Commodity and Equity Contracts | Equity
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		365	50
Derivatives not designated as hedging instruments | Other contracts | Stable value contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		26,313	26,653
Derivatives not designated as hedging instruments | Other contracts | Deferred value awards
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		497	473
Additional compensation and employee benefits expense due to accelerated vesting	$ 249
Derivatives designated as hedging instruments | Interest-rate contracts | Swap agreements
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		10,994	11,047
Derivatives designated as hedging instruments | Foreign exchange contracts | Forward and swap
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments		$ 3,884	$ 28,913