Debt - Revolving Credit and Letter of Credit (Details)	3 Months Ended	9 Months Ended
	Dec. 31, 2019 USD ($) Banks	Sep. 30, 2019 USD ($) Banks	Sep. 30, 2018 USD ($)
Line of Credit Facility [Line Items]
Debt covenant compliance		The Company was in compliance with its debt covenants as of September 30, 2019.
Repayment of long-term debt		$ 3,340,000,000	$ 131,000,000
Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks		23
Maximum borrowing capacity		$ 3,250,000,000
Amount by which credit facilty amount can be increased		$ 500,000,000
Amount by which credit facilty term length can be increased		1 year
Leverage ratio covenant		60.00%
Revolving Credit Facility [Member] | 364-day Revolving Credit Agreement [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks	23
Credit agreement term	364 days
Maximum borrowing capacity	$ 1,000,000,000.0
Revolving credit agreement term-out option maturity period	1 year
Letter of Credit [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 500,000,000
Letters of credit outstanding		21,000,000
Term Loan Credit Agreement [Member]
Line of Credit Facility [Line Items]
Aggregate principal amount		3,000,000,000.0
Remaining amount available for borrowing		$ 0
Leverage ratio covenant		60.00%
Repayment of long-term debt		$ 2,000,000,000.0