Summary of Significant Accounting Policies - Schedule of Changes in Restructuring Reserve (Parenthetical) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	$ 7,300,000	$ 15,500,000
Severance costs	74,200,000	32,400,000
Office closure costs	27,300,000	9,600,000
Other costs	9,200,000
Costs paid or utilized	71,900,000	50,200,000
Balance at end of year	46,100,000	7,300,000
Americas | United States
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	300,000	300,000
Severance costs	3,800,000	1,300,000
Office closure costs	17,800,000	5,400,000
Other costs	4,200,000
Costs paid or utilized	24,700,000	6,700,000
Balance at end of year	1,400,000	300,000
Southern Europe | France
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0	1,100,000
Office closure costs	2,600,000
Other costs	1,200,000
Costs paid or utilized	3,200,000	1,100,000
Balance at end of year	600,000	0
Southern Europe | Italy
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	300,000	500,000
Severance costs	1,900,000	2,300,000
Office closure costs	500,000
Other costs	1,000,000.0
Costs paid or utilized	2,300,000	2,500,000
Balance at end of year	$ 1,400,000	$ 300,000