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Financial Instruments - Available-For-Sale Securities (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Sales of available-for-sale securities        
Gross realized gains (before taxes) - new guidance    
Gross realized losses (before taxes) - new guidance    
Gross realized gains (before taxes) - prior year guidance    
Gross realized losses (before taxes) - prior year guidance    
Net changes related to available-for-sale securities, net of tax:        
Net unrealized gains/(losses) arising during the period
Maximum contractual maturities of substantially all available-for-sale debt securities     1 year