Business Segment Information - Segment Operating Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Total segment operating income	$ 309,515	$ 47,565	$ 572,952	$ 525,388
Operating segments
Segment Reporting Information [Line Items]
Total segment operating income	671,685	616,664	1,166,951	1,088,498
Intersegment eliminations
Segment Reporting Information [Line Items]
Total segment operating income	328	(249)	(579)	(556)
Pharmaceutical Distribution Services | Operating segments
Segment Reporting Information [Line Items]
Total segment operating income	563,097	517,034	954,791	890,241
Other | Operating segments
Segment Reporting Information [Line Items]
Total segment operating income	$ 108,260	$ 99,879	$ 212,739	$ 198,813