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Financial Instruments - Available-For-Sale Securities (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Sales of available-for-sale securities    
Proceeds - new guidance $ 0  
Proceeds - prior year guidance   $ 5
Gross realized gains (before taxes) - prior year guidance   1
Gross realized losses (before taxes) - prior year guidance   2
Unrealized holding gains/(losses) on available-for-sale securities    
Net unrealized gains/(losses) arising during the period (2) (1)
Net unrealized (gains)/losses reclassified to net income 0 1
Writedowns included in net income for the period $ 0 $ 0
Maximum contractual maturities of substantially all available-for-sale debt securities 1 year