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Fair Value Measurements (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
6.7% Senior Notes | UScellular    
Financial Instruments    
Interest rate 6.70%  
Book Value | Retail    
Financial Instruments    
Long-term debt $ 1,500 $ 2,753
Book Value | Institutional    
Financial Instruments    
Long-term debt 535 535
Book Value | Other    
Financial Instruments    
Long-term debt 820 230
Fair Value | Level 2 | Retail    
Financial Instruments    
Long-term debt 1,604 2,809
Fair Value | Level 2 | Institutional    
Financial Instruments    
Long-term debt 679 707
Fair Value | Level 2 | Other    
Financial Instruments    
Long-term debt $ 820 $ 230
Interest rate | Institutional and Other | Minimum    
Financial Instruments    
Fair value assumption, interest rate 1.24% 1.35%
Interest rate | Institutional and Other | Maximum    
Financial Instruments    
Fair value assumption, interest rate 4.25% 3.75%