Business & Basis of Presentation (Details) $ / shares in Units, $ in Billions	6 Months Ended	9 Months Ended
	Mar. 31, 2019 USD ($) shares	Sep. 30, 2018 segment satellite_radio_system $ / shares	Sep. 23, 2018 $ / shares	Dec. 31, 2017 $ / shares
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of satellite radio systems | satellite_radio_system		2
Business Combinations [Abstract]
Common stock, par value (in dollars per share)		$ 0.001	$ 0.001	$ 0.001
Segment Reporting, Disclosure of Entity's Reportable Segments [Abstract]
Number of reportable segments | segment		1
Common Stock | Management | Liberty Media
Related Party Transactions [Abstract]
Related party ownership percentage		71.00%
Pandora
Business Combinations [Abstract]
Common stock, par value (in dollars per share)			$ 0.0001
Pandora | Forecast
Business Combinations [Abstract]
Agreement value | $	$ 3.5
Shares issuable per acquiree share (in shares) | shares	1.44
Minimum
Related Party Transaction [Line Items]
Length of prepaid subscriptions, term		3 months
Maximum
Related Party Transaction [Line Items]
Length of prepaid subscriptions, term		12 months