Leases - Schedule of Balances and Classification of Operating Lease Right-of-Use Assets and Operating Lease Liabilities within Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets:
Operating lease right-of-use assets	$ 187,828	$ 186,998
Total assets	187,828	186,998
Liabilities:
Current operating lease liabilities	$ 43,097	$ 43,542
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	bro:AccruedExpenseAndOtherLiabilityMember	bro:AccruedExpenseAndOtherLiabilityMember
Non-current operating lease liabilities	$ 171,571	$ 172,935
Total liabilities	$ 214,668	$ 216,477