Leases - Summary of Supplemental Balance Sheet Information Related to Leases (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2019
Leases [Abstract]
Lease right-of-use assets	$ 373.9	$ 393.5	$ 379.6
Other current lease liabilities	$ 88.8
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	ajg:AccruedCompensationAndOtherAccruedLiabilities
Lease liabilities	$ 320.9	$ 340.9
Total lease liabilities	$ 409.7		$ 420.3
Weighted-average remaining lease term, years	5 years 3 months 18 days
Weighted-average discount rate	3.60%