Commitments and Contingencies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Commitments [Line Items]
Office space and equipment under non-cancelable lease agreements, expiration through year	2033
Rental expense relating to operating leases	$ 84,632,000	$ 76,704,000	$ 79,549,000
Office space under sublease agreements, expiration through year	2022
Sublease income	$ 7,659,000	7,858,000	9,587,000
Capital lease obligation paid through	2017
Book value of assets under capital lease		$ 13,628,000
Guarantees indemnifications	$ 2,995,000
Collateral/counter-guarantees	2,995,000
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	0
Business Acquisitions [Member] | Class A Common Stock [Member]
Other Commitments [Line Items]
Common stock issued and issuable		60,817
Contingent shares issuable upon satisfaction of performance thresholds		810,742
Business Acquisitions [Member] | Class A Common Stock [Member] | Non-Contingent [Member]
Other Commitments [Line Items]
Common stock issued and issuable		204,651
Buildings [Member]
Other Commitments [Line Items]
Net book value of certain buildings that primarily collateralized capital lease obligations		$ 13,538,000
LMDC Holdings [Member]
Other Commitments [Line Items]
Sublease income			$ 1,281,000