Business Segment Information - Reconciliation of Segment Operating Income to Income (Loss) from Operations before Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Operating income	$ 309,515	$ 47,565	$ 572,952	$ 525,388
Gain from antitrust litigation settlements	100	52,000	8,500	139,300
LIFO (expense) credit			(37,134)	69,834
Employee severance, litigation, and other	(67,732)	(55,389)	(107,041)	(96,061)
Impairment of PharMEDium assets	(223,652)	(570,000)	(361,652)	(570,000)
Other (income) loss	(1,109)	(14,494)	1,733	(11,397)
Interest expense, net	34,421	43,275	65,428	85,445
Income before income taxes	276,203	18,784	505,791	451,340
Operating segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Operating income	671,685	616,664	1,166,951	1,088,498
Segment reconciling items
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Gain from antitrust litigation settlements	54	51,976	8,546	139,255
LIFO (expense) credit	(23,853)	66,805	(37,134)	69,834
PharMEDium remediation costs	0	(15,897)	(16,165)	(36,392)
PharMEDium shutdown costs	(32,470)	0	(32,470)	0
New York State Opioid Stewardship Act	0	0	0	22,000
Contingent consideration adjustment	12,153	0	12,153	0
Acquisition-related intangibles amortization	(26,670)	(46,594)	(60,236)	(91,746)
Employee severance, litigation, and other	(67,732)	(55,389)	(107,041)	(96,061)
Impairment of PharMEDium assets	$ (223,652)	$ (570,000)	$ (361,652)	$ (570,000)