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Background (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2017
USD ($)
Mar. 31, 2018
USD ($)
Geographic Areas, Revenues from External Customers [Abstract]      
Number of states in which entity operates (states)     14
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Provisional income tax expense (benefit)   $ (555)  
Accumulated deficit   (713) $ (505)
Impact of 606 | Accounting Standards Update 2014-09      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Accumulated deficit   $ 128 $ (138)
Business Customer      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Deferred Customer Contract, Amortization Period 49 months    
Consumer Customers      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Deferred Customer Contract, Amortization Period 30 months