Leases - Schedule of Balances and Classification of Operating Lease Right-of-Use Assets and Operating Lease Liabilities within Condensed Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Operating lease right-of-use assets	$ 186,998	$ 184,288
Total assets	186,998	184,288
Liabilities:
Current operating lease liabilities	$ 43,542	$ 43,415
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	bro:AccruedExpensesAndOtherLiabilitiesMember	bro:AccruedExpensesAndOtherLiabilitiesMember
Non-current operating lease liabilities	$ 172,935	$ 167,855
Total liabilities	$ 216,477	$ 211,270