Business Combinations - Summary of Estimated Fair Values of Net Assets Acquired (Detail) $ in Millions	Dec. 31, 2016 USD ($)
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	$ 13.3
Other current assets	67.2
Fixed assets	3.6
Noncurrent assets	1.5
Goodwill	225.4
Expiration lists	218.1
Non-compete agreements	2.8
Trade names	0.4
Total assets acquired	532.3
Current liabilities	67.0
Noncurrent liabilities	65.6
Total liabilities assumed	132.6
Total net assets acquired	399.7
Kane's Insurance Management Operations (KIM) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	2.2
Other current assets	1.8
Fixed assets	0.4
Noncurrent assets	1.2
Goodwill	9.2
Expiration lists	20.0
Total assets acquired	34.8
Current liabilities	3.9
Noncurrent liabilities	0.1
Total liabilities assumed	4.0
Total net assets acquired	30.8
KDC Associates, LLC (KDC) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	0.3
Other current assets	9.3
Fixed assets	0.1
Goodwill	12.1
Expiration lists	13.8
Non-compete agreements	0.1
Total assets acquired	35.7
Current liabilities	8.9
Noncurrent liabilities	0.4
Total liabilities assumed	9.3
Total net assets acquired	26.4
McNeary, Inc. (MNI) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	3.0
Other current assets	1.7
Fixed assets	0.1
Goodwill	19.5
Expiration lists	13.8
Non-compete agreements	0.1
Trade names	0.1
Total assets acquired	38.3
Current liabilities	2.2
Noncurrent liabilities	8.7
Total liabilities assumed	10.9
Total net assets acquired	27.4
Brim AB [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	0.5
Other current assets	36.0
Fixed assets	0.6
Goodwill	19.7
Expiration lists	14.6
Non-compete agreements	0.6
Trade names	0.3
Total assets acquired	72.3
Current liabilities	33.2
Noncurrent liabilities	9.1
Total liabilities assumed	42.3
Total net assets acquired	30.0
Victory Insurance Agency Inc [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	0.2
Other current assets	2.1
Fixed assets	0.1
Goodwill	18.5
Expiration lists	11.3
Non-compete agreements	0.4
Total assets acquired	32.6
Current liabilities	2.2
Noncurrent liabilities	4.6
Total liabilities assumed	6.8
Total net assets acquired	25.8
Altman & Cronin Benefit Consultants, LLC (ACB) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Other current assets	1.3
Goodwill	19.9
Expiration lists	18.9
Non-compete agreements	0.4
Total assets acquired	40.5
Total net assets acquired	40.5
Regency Insurance Group, Inc. (RIG) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	0.6
Other current assets	0.6
Goodwill	10.5
Expiration lists	9.3
Non-compete agreements	0.2
Total assets acquired	21.2
Current liabilities	1.0
Total liabilities assumed	1.0
Total net assets acquired	20.2
Argentis (ARG) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	1.4
Other current assets	0.4
Noncurrent assets	0.3
Goodwill	12.1
Expiration lists	24.1
Total assets acquired	38.3
Current liabilities	0.8
Noncurrent liabilities	15.0
Total liabilities assumed	15.8
Total net assets acquired	22.5
Twenty-nine Other Acquisitions [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	5.1
Other current assets	14.0
Fixed assets	2.3
Goodwill	103.9
Expiration lists	92.3
Non-compete agreements	1.0
Total assets acquired	218.6
Current liabilities	14.8
Noncurrent liabilities	27.7
Total liabilities assumed	42.5
Total net assets acquired	$ 176.1