Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2015	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Litigation Settlement, Amount					$ 398,000,000
Increase (Decrease) in Restricted Cash					$ 335,000,000
Restricted Investments, Current	$ 30,000,000		$ 30,000,000			$ 30,000,000
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities	0	$ 0	0	$ 0
Other financial instruments [Abstract]
Long-term Debt, Carrying Value	2,613,000,000		2,613,000,000			2,615,000,000
Long-term Debt, Fair Value	$ 2,710,000,000		$ 2,710,000,000			$ 2,768,000,000