Business Segment Information - Reconciliation of Segment Revenue, Operating Income, Assets, Depreciation and Amortization, and Capital Expenditures (Details) - USD ($) $ in Thousands	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
Segment Reporting Information [Line Items]
Revenue	$ 49,244,768	$ 45,366,777	$ 47,417,639	$ 47,864,742	$ 45,637,802	$ 45,239,265	$ 43,319,602	$ 45,392,452	$ 189,893,926	$ 179,589,121	$ 167,939,635
Operating income	(6,113,137)	$ 404,831	$ 309,515	$ 263,437	179,841	$ 406,694	$ 47,565	$ 477,823	(5,135,354)	1,111,923	1,443,685
Assets	$ 44,274,830				$ 39,171,980				44,274,830	39,171,980
Depreciation and amortization									391,062	462,407	465,127
Capital expenditures									369,677	310,222	336,411
Pharmaceutical Distribution
Segment Reporting Information [Line Items]
Capital expenditures									201,144	210,161	190,191
Other
Segment Reporting Information [Line Items]
Capital expenditures									168,533	100,061	146,220
Operating segments
Segment Reporting Information [Line Items]
Operating income									2,204,447	2,051,252	1,981,230
Operating segments | Pharmaceutical Distribution
Segment Reporting Information [Line Items]
Revenue									182,467,189	172,813,537	161,699,343
Operating income									1,807,001	1,671,251	1,626,748
Depreciation and amortization									203,062	232,735	225,608
Operating segments | MWI Animal Health
Segment Reporting Information [Line Items]
Revenue									4,216,462	3,975,232	3,789,759
Operating segments | Global Commercialization Services
Segment Reporting Information [Line Items]
Revenue									3,308,640	2,893,109	2,542,971
Operating segments | Other
Segment Reporting Information [Line Items]
Revenue									7,525,102	6,868,341	6,332,730
Operating income									400,139	380,660	355,091
Depreciation and amortization									77,522	69,824	64,768
Intersegment eliminations
Segment Reporting Information [Line Items]
Revenue									(98,365)	(92,757)	(92,438)
Operating income									(2,693)	(659)	(609)
Segment reconciling items
Segment Reporting Information [Line Items]
Acquisition-related intangibles amortization									$ 110,478	$ 159,848	$ 174,751