Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Jun. 30, 2019
Fair Value of Financial Instruments [Line Items]
Investment excludes receivables, payables and accrued income	$ 4	$ (15)
Cash balance excluded from available for sale securities	20	(2)
Investments not required to be disclosed	$ 253	$ 275
NUG contracts
Fair Value of Financial Instruments [Line Items]
Period of future observable data to determine contract price	2 years