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Investments - Investments - VIEs and Other Collateral Transactions (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Concentration Risk [Line Items]      
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount $ 1,000 $ 1,000  
Fair value of securities on deposit 2,300   $ 2,200
Variable Interest Entity, Not Primary Beneficiary [Member]      
Concentration Risk [Line Items]      
Variable Interest Entity, Reporting Entity Involvement, Commitments 767 718  
US Treasury Securities [Member]      
Concentration Risk [Line Items]      
Debt Securities, Available-for-sale, Restricted $ 35 $ 47