Fair Value Measurements (Details Textuals) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Fair Value of Financial Instruments [Line Items]
Investment excludes receivables, payables and accrued income	$ (12)	$ (11)
Cash balance excluded from available for sale securities	5	11
Investments not required to be disclosed	$ 255	$ 255
NUG contracts
Fair Value of Financial Instruments [Line Items]
Period of future observable data to determine contract price	2 years