Contracts with Customers - Summary of Expected Revenue Related to Performance Obligations (Detail) $ in Millions	Mar. 31, 2026 USD ($)
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 986
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2026-04-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 835
Remaining performance obligation, expected timing of satisfaction, period	9 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2027-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 98
Remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2028-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 25
Remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2029-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 11
Remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2030-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 5
Remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2031-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 12
Remaining performance obligation, expected timing of satisfaction, period
Brokerage
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 778
Brokerage \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2026-04-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 716
Remaining performance obligation, expected timing of satisfaction, period	9 months
Brokerage \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2027-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 56
Remaining performance obligation, expected timing of satisfaction, period	1 year
Brokerage \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2028-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 5
Remaining performance obligation, expected timing of satisfaction, period	1 year
Brokerage \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2029-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 1
Remaining performance obligation, expected timing of satisfaction, period	1 year
Brokerage \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2030-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 0
Remaining performance obligation, expected timing of satisfaction, period	1 year
Brokerage \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2031-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 0
Remaining performance obligation, expected timing of satisfaction, period
Risk Management
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 208
Risk Management \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2026-04-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 119
Remaining performance obligation, expected timing of satisfaction, period	9 months
Risk Management \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2027-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 42
Remaining performance obligation, expected timing of satisfaction, period	1 year
Risk Management \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2028-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 20
Remaining performance obligation, expected timing of satisfaction, period	1 year
Risk Management \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2029-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 10
Remaining performance obligation, expected timing of satisfaction, period	1 year
Risk Management \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2030-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 5
Remaining performance obligation, expected timing of satisfaction, period	1 year
Risk Management \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2031-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 12
Remaining performance obligation, expected timing of satisfaction, period

Arthur J. Gallagher & Co. 10-Q