Property, Plant, and Equipment (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Jan. 31, 2019	Mar. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Proceeds from sale of property				$ 106	$ 106
Towers [Member]
Property, Plant and Equipment [Line Items]
Proceeds from sale of property	$ 76
Aggregate carrying value	1
Gain on sale of property	$ 75
Accounting Standards Update 2016-02 [Member]
Property, Plant and Equipment [Line Items]
Cumulative-effect adjustment, Net of tax			$ 15
Cumulative-effect adjustment		$ 11	11
Accounting Standards Update 2018-11 [Member]
Property, Plant and Equipment [Line Items]
Cumulative-effect adjustment, Net of tax			15
Cumulative-effect adjustment			$ 11