Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Details) - USD ($) $ in Millions			9 Months Ended
	Aug. 01, 2025	Jun. 10, 2025	Sep. 30, 2025	Sep. 30, 2024
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid			$ 8,514
Common stock issued			612
Other payable			699
Recorded earn-out payable			24	$ 28
Total consideration			9,849
Maximum potential earn-out payable			48
Allocation of purchase price:
Cash and equivalents			319
Fiduciary cash			536
Fiduciary receivables			460
Other current assets			1,135
Goodwill			6,703
Other assets			117
Total assets acquired			12,577
Fiduciary liabilities			(993)
Other current liabilities			(564)
Deferred income tax, net			(113)
Other long-term liabilities			(1,058)
Total liabilities assumed			(2,728)
Net assets acquired			9,849
Purchased Customer Accounts and Other
Allocation of purchase price:
Intangible assets			3,307
NBS Insurance Agency | Specialty Distribution
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid			54
Common stock issued			0
Other payable			0
Recorded earn-out payable			0
Total consideration			54
Maximum potential earn-out payable			0
Allocation of purchase price:
Cash and equivalents			(3)
Fiduciary cash			14
Fiduciary receivables			(1)
Other current assets			4
Goodwill			34
Other assets			0
Total assets acquired			64
Fiduciary liabilities			(10)
Other current liabilities			0
Deferred income tax, net			0
Other long-term liabilities			0
Total liabilities assumed			(10)
Net assets acquired			54
NBS Insurance Agency | Specialty Distribution | Purchased Customer Accounts and Other
Allocation of purchase price:
Intangible assets			16
Tim Parkman, Inc. | Specialty Distribution
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid			69
Common stock issued			0
Other payable			6
Recorded earn-out payable			4
Total consideration			79
Maximum potential earn-out payable			23
Allocation of purchase price:
Cash and equivalents			0
Fiduciary cash			0
Fiduciary receivables			0
Other current assets			0
Goodwill			60
Other assets			0
Total assets acquired			79
Fiduciary liabilities			0
Other current liabilities			0
Deferred income tax, net			0
Other long-term liabilities			0
Total liabilities assumed			0
Net assets acquired			79
Tim Parkman, Inc. | Specialty Distribution | Purchased Customer Accounts and Other
Allocation of purchase price:
Intangible assets			19
RSC Topco, Inc.
Fair Value Of Assets And Liabilities Statement [Line Items]
Common stock issued	$ 612	$ 1,045
RSC Topco, Inc. | Retail & Specialty Distribution
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid			8,293
Common stock issued			612
Other payable			693
Recorded earn-out payable			0
Total consideration			9,598
Allocation of purchase price:
Cash and equivalents			316
Fiduciary cash			516
Fiduciary receivables			437
Other current assets			1,129
Goodwill			6,538
Other assets			112
Total assets acquired			12,284
Fiduciary liabilities			(957)
Other current liabilities			(559)
Deferred income tax, net			(114)
Other long-term liabilities			(1,056)
Total liabilities assumed			(2,686)
Net assets acquired			9,598
RSC Topco, Inc. | Retail & Specialty Distribution | Purchased Customer Accounts and Other
Allocation of purchase price:
Intangible assets			3,236
Other Acquisitions | Retail & Specialty Distribution
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid			98
Common stock issued			0
Other payable			0
Recorded earn-out payable			20
Total consideration			118
Maximum potential earn-out payable			25
Allocation of purchase price:
Cash and equivalents			6
Fiduciary cash			6
Fiduciary receivables			24
Other current assets			2
Goodwill			71
Other assets			5
Total assets acquired			150
Fiduciary liabilities			(26)
Other current liabilities			(5)
Deferred income tax, net			1
Other long-term liabilities			(2)
Total liabilities assumed			(32)
Net assets acquired			118
Other Acquisitions | Retail & Specialty Distribution | Purchased Customer Accounts and Other
Allocation of purchase price:
Intangible assets			$ 36