Derivative Financial Instruments - Schedule of Outstanding Hedges: (Notional Amount) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivatives not designated as hedging instruments | Interest rate contracts | Futures
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments	$ 4,387	$ 2,348
Derivatives not designated as hedging instruments | Foreign exchange contracts | Futures
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments	414	49
Derivatives not designated as hedging instruments | Foreign exchange contracts | Forward, swap and spot
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments	2,284,676	2,238,819
Derivatives not designated as hedging instruments | Foreign exchange contracts | Options purchased
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments	797	578
Derivatives not designated as hedging instruments | Foreign exchange contracts | Options written
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments	505	576
Derivatives not designated as hedging instruments | Other derivative contracts | Stable value contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments	26,991	26,634
Derivatives not designated as hedging instruments | Other derivative contracts | Deferred value awards
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments	559	434
Hedged Items Currently Designated | Interest rate contracts | Swap agreements
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments	10,596	10,596
Hedged Items Currently Designated | Foreign exchange contracts | Forward and swap
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of derivative instruments	$ 3,050	$ 3,412