Business Combinations - Summary of Estimated Fair Values of Net Assets Acquired (Detail) $ in Millions	Dec. 31, 2021 USD ($)
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	$ 840.1
Premiums and fees receivable	5,619.3
Other current assets	67.1
Fixed assets	63.6
Noncurrent assets	33.2
Goodwill	2,561.5
Expiration lists	2,027.8
Non-compete agreements	7.3
Trade names	5.2
Total assets acquired	11,225.1
Premiums payables to underwriting enterprises	6,154.7
Other current liabilities	247.5
Noncurrent liabilities	151.4
Total liabilities assumed	6,553.6
Total net assets acquired	4,671.5
Atlas General Holdings LLC [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	32.8
Premiums and fees receivable	111.4
Other current assets	0.6
Fixed assets	7.4
Noncurrent assets	1.2
Goodwill	45.9
Expiration lists	39.7
Non-compete agreements	0.3
Total assets acquired	239.3
Premiums payables to underwriting enterprises	125.2
Other current liabilities	4.6
Noncurrent liabilities	0.2
Total liabilities assumed	130.0
Total net assets acquired	109.3
Bollington Wilson Group [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	24.9
Premiums and fees receivable	8.7
Other current assets	6.6
Fixed assets	3.8
Noncurrent assets	5.3
Goodwill	257.1
Expiration lists	81.6
Non-compete agreements	1.2
Trade names	3.4
Total assets acquired	392.6
Premiums payables to underwriting enterprises	18.5
Other current liabilities	21.1
Noncurrent liabilities	23.6
Total liabilities assumed	63.2
Total net assets acquired	329.4
LDJ American Online Benefits Group, LLC [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	0.7
Premiums and fees receivable	7.8
Other current assets	8.0
Noncurrent assets	0.2
Goodwill	25.7
Expiration lists	25.4
Non-compete agreements	0.2
Trade names	0.2
Total assets acquired	68.2
Premiums payables to underwriting enterprises	6.4
Other current liabilities	1.6
Noncurrent liabilities	0.1
Total liabilities assumed	8.1
Total net assets acquired	60.1
Edelweiss Gallagher Insurance Brokers Limited [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	2.7
Premiums and fees receivable	8.2
Other current assets	3.7
Fixed assets	0.3
Noncurrent assets	3.1
Goodwill	40.3
Expiration lists	24.3
Non-compete agreements	0.5
Total assets acquired	83.1
Premiums payables to underwriting enterprises	2.9
Other current liabilities	3.5
Noncurrent liabilities	27.2
Total liabilities assumed	33.6
Total net assets acquired	49.5
Manchester Underwriting Agencies Limited [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	5.8
Premiums and fees receivable	4.0
Other current assets	2.4
Fixed assets	0.3
Noncurrent assets	0.5
Goodwill	32.1
Expiration lists	17.5
Non-compete agreements	1.5
Trade names	0.3
Total assets acquired	64.4
Premiums payables to underwriting enterprises	1.5
Other current liabilities	2.4
Noncurrent liabilities	5.7
Total liabilities assumed	9.6
Total net assets acquired	54.8
Willis Reinsurance Operations [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	748.7
Premiums and fees receivable	5,410.8
Other current assets	41.4
Fixed assets	51.5
Noncurrent assets	14.6
Goodwill	1,930.8
Expiration lists	1,588.2
Total assets acquired	9,786.0
Premiums payables to underwriting enterprises	5,936.4
Other current liabilities	197.8
Noncurrent liabilities	72.9
Total liabilities assumed	6,207.1
Total net assets acquired	3,578.9
Thirty-Two Other Acquisitions [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	24.5
Premiums and fees receivable	68.4
Other current assets	4.4
Fixed assets	0.3
Noncurrent assets	8.3
Goodwill	229.6
Expiration lists	251.1
Non-compete agreements	3.6
Trade names	1.3
Total assets acquired	591.5
Premiums payables to underwriting enterprises	63.8
Other current liabilities	16.5
Noncurrent liabilities	21.7
Total liabilities assumed	102.0
Total net assets acquired	$ 489.5