Summary of Significant Accounting Policies (Interest Expense and Amortization of Deferred Financing Costs) (Details) - USD ($) $ in Millions	3 Months Ended							6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Interest Expense, Debt, Excluding Amortization												$ 682	$ 635	$ 582
Amortization of deferred financing costs and adjustments on long-term debt, net												21	21	19
Capitalized Interest												(20)	(15)	(12)
Other												0	1	2
Interest Expense, Debt	$ (173)	$ (169)	$ (168)	$ (164)	$ (160)	$ (158)	$ (160)	$ (337)	$ (318)	$ (510)	$ (478)	$ 683	$ 642	$ 591