Acquisitions and Dispositions - Acquired Entities (Details) - USD ($)		9 Months Ended		12 Months Ended
	Apr. 01, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Mar. 09, 2020
Business Acquisition [Line Items]
G4S intercompany payments		$ 50,600,000	$ (134,200,000)
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Restricted cash		321,700,000		$ 322,000,000.0
Right-of-use assets, net		304,100,000		322,000,000.0
Goodwill		1,426,600,000		1,219,200,000
Fair value of noncontrolling interest		$ (116,400,000)
PAI
Business Acquisition [Line Items]
Acquisition date	Apr. 01, 2021
Percentage of shares acquired	100.00%
Annual revenues				$ 95,000,000
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Purchase consideration - cash paid	$ 215,500,000
Fair value of purchase consideration	215,500,000
Cash	12,300,000
Accounts receivable	7,300,000
Other current assets	5,100,000
Property and equipment, net	15,000,000.0
Intangible assets	95,000,000.0
Goodwill	127,400,000
Other noncurrent assets	4,500,000
Current liabilities	(41,200,000)
Noncurrent liabilities	(9,900,000)
Fair value of net assets acquired	215,500,000
G4Si
Business Acquisition [Line Items]
Percentage of shares acquired						100.00%
G4S
Business Acquisition [Line Items]
Percentage of shares acquired		100.00%
G4S intercompany payments		$ 114,000,000
Annual revenues					$ 800,000,000
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Purchase consideration - cash paid		817,400,000
Contingent consideration		22,000,000.0
Liabilities assumed from seller		2,900,000
Indemnification asset		(5,200,000)
Fair value of purchase consideration		837,100,000
Cash		244,400,000
Restricted cash		30,100,000
Accounts receivable		145,800,000
Other current assets		30,800,000
Property and equipment, net		123,800,000
Right-of-use assets, net		77,500,000
Intangible assets		206,000,000.0
Goodwill		535,900,000
Other noncurrent assets		16,200,000
Current liabilities		(296,200,000)
Lease liabilities		(68,100,000)
Noncurrent liabilities		(92,700,000)
Fair value of net assets acquired		$ 953,500,000
G4S Tranche III
Business Acquisition [Line Items]
Percentage of shares acquired		100.00%
Customer relationships | PAI
Business Acquisition [Line Items]
Remaining Amortization Period		10 years
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Intangible assets	60,000,000
Customer relationships | G4S
Business Acquisition [Line Items]
Remaining Amortization Period		15 years
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Intangible assets		$ 206,000,000
Developed technology | PAI
Business Acquisition [Line Items]
Remaining Amortization Period		12 years
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Intangible assets	26,000,000
Trade names | PAI
Business Acquisition [Line Items]
Remaining Amortization Period		5 years
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Intangible assets	$ 9,000,000
North America | PAI
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill		$ 2,000,000
Europe | G4S
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill		191,000,000
Rest of World | G4S
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill		342,000,000
Latin America | G4S
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill		$ 3,000,000