Summary of Significant Accounting Policies - Schedule of Changes in Restructuring Reserve (Parenthetical) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	$ 15,500,000	$ 13,500,000
Severance costs	32,400,000	31,800,000
Office closure costs	9,600,000	7,500,000
Costs paid or utilized	50,200,000	37,300,000
Balance at end of year	7,300,000	15,500,000
Operating Lease Right-of-Use Asset
Restructuring Cost and Reserve [Line Items]
Office closure costs	9,600,000
Operating Lease Liabilities, Current
Restructuring Cost and Reserve [Line Items]
Office closure costs	3,200,000
Americas | United States
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	300,000	1,500,000
Severance costs	1,100,000
Office closure costs	1,300,000
Costs paid or utilized	2,400,000	1,200,000
Balance at end of year	300,000	300,000
Southern Europe | France
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	900,000	900,000
Costs paid or utilized	900,000
Balance at end of year	0	900,000
Southern Europe | Italy
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	500,000	0
Severance costs	2,300,000	1,900,000
Office closure costs	2,500,000
Costs paid or utilized	$ 300,000	1,400,000
Balance at end of year		$ 500,000