Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance [Line Items]
Provision for doubtful accounts		$ (8)	$ (11)	$ (11)
Allowance for Doubtful Accounts, Current [Member]
Valuation Allowance [Line Items]
Balance at Beginning of Period		41	48	196
Valuation Allowances and Reserves, Adjustments		0	0	(162)
Valuation Allowances and Reserves, Deductions		(18)	(18)	3
Balance at End of Period		51	41	48
OpCo [Member] | Allowance for Doubtful Accounts, Current [Member]
Valuation Allowance [Line Items]
Valuation Allowances and Reserves, Adjustments	[1]	$ 20	$ 0	$ 0

[1] ____________________(1) See Note 4 for further details relating to the acquisition of OpCo.