v3.20.4
Summary of significant accounting policies - Concentrations of credit risk (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Total Revenue | Entities affiliated with T-Mobile          
Concentrations of credit risk          
Concentration risk percentage     21.00% 20.00% 26.00%
Total Revenue | Entities affiliated with AT&T Inc.          
Concentrations of credit risk          
Concentration risk percentage     13.00% 12.00%  
Total Revenue | Entities affiliated with Verizon Communications Inc.          
Concentrations of credit risk          
Concentration risk percentage     11.00% 11.00% 11.00%
Total accounts receivable | Entities affiliated with T-Mobile          
Concentrations of credit risk          
Concentration risk percentage 13.00% 13.00%      
Total accounts receivable | Entities affiliated with AT&T Inc.          
Concentrations of credit risk          
Concentration risk percentage 27.00% 34.00%      
Total accounts receivable | Entities affiliated with Verizon Communications Inc.          
Concentrations of credit risk          
Concentration risk percentage 11.00%