Summary of Significant Accounting Policies - Other Balance Sheet Information (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Accounts receivable allowance for doubtful accounts	$ 28	$ 32
Property, plant and equipment accumulated depreciation	1,684	1,811
Intangible assets accumulated amortization	1,182	1,149
Out-of-market contracts accumulated amortization	$ 0	$ 37