Derivative Financial Instruments - Narrative (Details) $ in Millions	Jun. 30, 2019 USD ($) contract	Dec. 31, 2018 USD ($) contract	Jun. 30, 2018 contract
Derivative Instruments, Gain (Loss)
Derivative, mumber of instruments held | contract	24,898	26,170
Cash and securities pledged as collateral by counterparties	$ 32
Securities pledged as collateral to counterparties	2
Collateral posted under MNAs for contracts containing credit-risk-contingent features	0	$ 2
Cash and securities pledged in the form of margin deposits	$ 46
Derivatives designated as fair value accounting hedging instruments
Derivative Instruments, Gain (Loss)
Derivative, mumber of instruments held | contract	1		1
Number of foreign currency derivatives held | contract	0		1