Business Combinations - Summary of Estimated Fair Values of Net Assets Acquired (Detail) $ in Millions	Dec. 31, 2020 USD ($)
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	$ 34.0
Other current assets	88.0
Fixed assets	3.2
Noncurrent assets	27.6
Goodwill	359.6
Expiration lists	467.2
Non-compete agreements	11.4
Trade names	12.5
Total assets acquired	1,003.5
Current liabilities	103.6
Noncurrent liabilities	23.8
Total liabilities assumed	127.4
Total net assets acquired	876.1
CRB [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Noncurrent assets	7.6
Goodwill	108.4
Expiration lists	133.7
Non-compete agreements	2.9
Trade names	4.7
Total assets acquired	257.3
Total net assets acquired	257.3
HES [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	3.3
Other current assets	1.0
Noncurrent assets	0.8
Goodwill	19.1
Expiration lists	13.7
Non-compete agreements	0.1
Total assets acquired	38.0
Current liabilities	4.4
Noncurrent liabilities	0.5
Total liabilities assumed	4.9
Total net assets acquired	33.1
CRES [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	4.3
Other current assets	12.4
Goodwill	21.3
Expiration lists	13.9
Trade names	0.3
Total assets acquired	52.2
Current liabilities	15.7
Total liabilities assumed	15.7
Total net assets acquired	36.5
OTI [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	0.6
Other current assets	3.0
Fixed assets	0.9
Noncurrent assets	6.7
Goodwill	25.4
Expiration lists	18.3
Non-compete agreements	0.4
Total assets acquired	55.3
Current liabilities	5.2
Noncurrent liabilities	7.5
Total liabilities assumed	12.7
Total net assets acquired	42.6
CIA [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	4.2
Other current assets	28.8
Fixed assets	0.1
Noncurrent assets	0.9
Goodwill	58.9
Expiration lists	56.2
Non-compete agreements	0.8
Trade names	7.5
Total assets acquired	157.4
Current liabilities	28.1
Noncurrent liabilities	0.2
Total liabilities assumed	28.3
Total net assets acquired	129.1
HAI [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	2.3
Other current assets	9.0
Fixed assets	1.1
Noncurrent assets	6.4
Goodwill	45.7
Expiration lists	62.5
Non-compete agreements	0.7
Total assets acquired	127.7
Current liabilities	9.5
Noncurrent liabilities	6.4
Total liabilities assumed	15.9
Total net assets acquired	111.8
AT [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	6.0
Other current assets	8.6
Fixed assets	0.5
Noncurrent assets	2.1
Goodwill	14.8
Expiration lists	78.2
Non-compete agreements	5.8
Total assets acquired	116.0
Current liabilities	8.4
Noncurrent liabilities	2.1
Total liabilities assumed	10.5
Total net assets acquired	105.5
Twenty Other Acquisitions [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	13.3
Other current assets	25.2
Fixed assets	0.6
Noncurrent assets	3.1
Goodwill	66.0
Expiration lists	90.7
Non-compete agreements	0.7
Total assets acquired	199.6
Current liabilities	32.3
Noncurrent liabilities	7.1
Total liabilities assumed	39.4
Total net assets acquired	$ 160.2