Commitments and Contingencies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Commitments [Line Items]
Office space and equipment under non-cancelable lease agreements, expiration through year	2033
Rental expense relating to operating leases	$ 84,636,000	$ 84,632,000	$ 76,704,000
Office space under sublease agreements, expiration through year	2022
Sublease income	$ 7,519,000	$ 7,659,000	$ 7,858,000
Capital lease obligation paid through		2017
Guarantees indemnifications	4,991,000
Collateral/counter-guarantees	4,947,000
LFB [Member]
Other Commitments [Line Items]
Other commitments	0
LFNY [Member]
Other Commitments [Line Items]
Other commitments	$ 0
Business Acquisitions [Member]
Other Commitments [Line Items]
Number of contingent additional shares earned	20,293
Business Acquisitions [Member] | Class A Common Stock [Member]
Other Commitments [Line Items]
Common stock issued and issuable			40,524
Business Acquisitions [Member] | Class A Common Stock [Member] | Non-Contingent [Member]
Other Commitments [Line Items]
Common stock issued and issuable			202,984
Business Acquisitions [Member] | Class A Common Stock [Member] | Maximum [Member]
Other Commitments [Line Items]
Contingent shares issuable upon satisfaction of performance thresholds			810,742