Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
6.7% Senior Notes | UScellular
Financial Instruments
Interest rate	6.70%
Book Value | Retail
Financial Instruments
Long-term debt	$ 2,153	$ 2,753
Book Value | Institutional
Financial Instruments
Long-term debt	535	535
Book Value | Other
Financial Instruments
Long-term debt	720	230
Fair Value | Level 2 | Retail
Financial Instruments
Long-term debt	2,276	2,809
Fair Value | Level 2 | Institutional
Financial Instruments
Long-term debt	670	707
Fair Value | Level 2 | Other
Financial Instruments
Long-term debt	$ 720	$ 230
Interest rate | Institutional and Other | Minimum
Financial Instruments
Fair value assumption, interest rate	1.26%	1.35%
Interest rate | Institutional and Other | Maximum
Financial Instruments
Fair value assumption, interest rate	4.34%	3.75%