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Commitments and Contingencies - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Jul. 29, 2015
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Commitments And Contingencies [Line Items]                        
Net lease and rental expense                   $ 80 $ 62 $ 60
Firm's expected commitments                   340 305  
Other off-balance sheet commitments to investees   $ 4       $ 3       4 3  
Revenues   $ 1,039 $ 958 $ 963 $ 1,045 $ 917 $ 917 $ 825 $ 917 $ 4,005 3,576 3,246
Sirius XM | Settled Litigation                        
Commitments And Contingencies [Line Items]                        
Receipts from plaintiff $ 210                      
Cash distribution                       33
Revenues         $ 1           $ 4 $ 28