Segment and Geographic Information - Total Revenues by Geographic Location (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Total revenues	$ 14,284.9	$ 8,243.4	$ 20,366.3	$ 11,245.0	$ 11,279.6	$ 7,873.0	$ 7,192.2	$ 6,552.2	$ 54,139.6	$ 32,897.0	$ 27,622.7
Operating revenues:	342.1	$ 322.6	$ 366.8	$ 276.8	286.9	$ 283.4	$ 271.1	$ 264.7	1,308.3	1,106.1	975.8
Long-lived assets, as defined:	62.1				43.9				62.1	43.9	42.4
United States [Member]
Total revenues									2,223.3	1,947.6	1,587.6
Operating revenues:									928.3	799.4	695.3
Long-lived assets, as defined:	55.4				33.9				55.4	33.9	33.0
Europe [Member]
Total revenues									532.6	280.2	189.6
Operating revenues:									237.9	209.6	189.0
Long-lived assets, as defined:	3.1				6.6				3.1	6.6	6.8
South America [Member]
Total revenues									58.9	56.5	59.5
Operating revenues:									58.9	56.5	58.0
Long-lived assets, as defined:	2.1				2.1				2.1	2.1	1.4
Middle East and Asia [Member]
Total revenues									51,317.1	30,606.9	25,781.4
Operating revenues:									75.5	34.8	28.9
Long-lived assets, as defined:	1.3				1.0				1.3	1.0	1.2
Other [Member]
Total revenues									7.7	5.8	4.6
Operating revenues:									7.7	5.8	4.6
Long-lived assets, as defined:	$ 0.2				$ 0.3				$ 0.2	$ 0.3	$ 0.0