Background (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) state segment	Dec. 31, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of states in which entity operates | state	14
Number of reportable segments | segment	1
Provisional income tax expense (benefit)		$ (555)
Provisional income tax expense (benefit), adjustment	$ 137
Business Customer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amortization period (in months)	49 months
Consumer Customers
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amortization period (in months)	30 months
Retained Earnings | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative net effect of adoption of ASU 2014-09, Revenue from Contracts with Customers		147	$ 19	$ 0	$ 0
Cumulative effect of new accounting principle in period of adoption, tax		$ 50	$ 7