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Other Investments (Details) - Greece [Member] - USD ($)
$ in Millions
3 Months Ended
Jun. 30, 2018
Mar. 31, 2018
Schedule of Equity Method Investments [Line Items]    
Payments to Acquire Equity Method Investments $ 18 $ 19
Equity Method Investment, Ownership Percentage 40.00% 20.00%
Equity Method Investment, Commitment to Purchase Ownership Percentage   20.00%
Commitment to Purchase within next 12 months [Member]    
Schedule of Equity Method Investments [Line Items]    
Equity Method Investment, Commitment to Purchase Ownership Percentage   10.00%
Commitment to Purchase in year 2 [Member]    
Schedule of Equity Method Investments [Line Items]    
Equity Method Investment, Commitment to Purchase Ownership Percentage   10.00%