Fair Value Measurements (Details Textuals) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Mar. 31, 2019
Fair Value of Financial Instruments [Line Items]
Investment excludes receivables, payables and accrued income	$ 4	$ (12)
Cash balance excluded from available for sale securities	20	4
Investments not required to be disclosed	$ 253	$ 257
NUG contracts
Fair Value of Financial Instruments [Line Items]
Period of future observable data to determine contract price	2 years