Financial Instruments - Additional Information (Detail)		12 Months Ended
	Aug. 04, 2014 USD ($)	Dec. 31, 2015 USD ($) Security Short_Sale	Dec. 31, 2014 USD ($) Security Short_Sale	Dec. 31, 2013 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets measured on recurring basis		$ 28,321,000	$ 61,159,000
Number of Transfer made out of Level 3 and into Level 2 | Security		1
Number of Transfer made out of Level 3 and into Level 1 | Security		1	2
Payments of contingent consideration obligation		$ 2,166,000	$ 1,000,000	$ 0
Contingent consideration payable		1,267,000
Other-than-temporary impairment losses		0	0	545,000
Other-than-temporary impairment losses, cost method investment		461,000
Financial instruments held for investment activities		$ 14,285,000	3,461,000
Number of short-sales | Short_Sale		2
Face value amount		$ 75,000,000	75,000,000
Interest expense		35,037,000	21,183,000	0
Realized and unrealized gains		9,954,000	10,451,000
Investment funds valued at net asset value		66,602,000	104,888,000
Marketable securities with unrealized loss position		0	0
Carrying value of investment		4,257,000
Other investments, at cost		6,539,000	7,000,000
Impairment losses		461,000	0	0
Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments, at cost			5,000,000
Proceeds from sale of corporate debt investment		0	1,428,000	0
Non-public equity security with a cost basis			428,000	2,390,000
Proceeds maturity of note receivable carried at cost				317,000
Gross Gain from sale of marketable securities			1,176,000
Non Public Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments, at cost		6,539,000	7,000,000
Available-for-Sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Proceeds from sale of marketable securities		191,000	$ 0	25,431,000
Gross Gain from sale of marketable securities		$ 91,000		698,000
Gross loss from sale of marketable securities				$ 32,000
Short Sales One [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of short-sales | Short_Sale			1
Face value amount			$ 75,000,000
Coupon rate of treasury securities		4.625%	7.25%
Securities maturity date		2016-11	2016-05
Short Sales Two [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Coupon rate of treasury securities		4.625%
Securities maturity date		2016-11
Short Sales [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of short-sales | Short_Sale			2
Face value amount		$ 200,000,000
Short Sales One Matures On November 2015 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face value amount			$ 100,000,000
Coupon rate of treasury securities			4.50%
Securities maturity date			2015-11
Short Sales Two Matures On February 2016 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face value amount			$ 100,000,000
Coupon rate of treasury securities			4.50%
Securities maturity date			2016-02
Two Non Public Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities		6,510,000	$ 6,148,000
U.S. Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest expense		$ 11,160,000	11,678,000
Hedge Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of fair value redeemable on 60 days or less		54.00%
Amount of fair value redeemable on 60 days or less		$ 31,452,000
Percentage of fair value redeemable between 90 and 180 days		46.00%
Amount of fair value redeemable between 90 and 180 days		$ 26,450,000
Amount of redemption for fair value hedging funds		$ 28,000,000
Hedge Funds [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Redeemable notice period on quarterly basis		90 days
Hedge Funds [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Redeemable notice period		60 days
Redeemable notice period on quarterly basis		180 days
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on a non-recurring basis		$ 0	0
Liabilities measured at fair value on a non-recurring basis		0	0
Lazard Capital Markets LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payments of contingent consideration obligation	$ 1,000,000	2,166,000
Decrease in carrying value of liability due to securities lending		637,000
Contingent consideration payable	$ 4,070,000
Estimated Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets measured on recurring basis		$ 16,505,000	$ 3,188,000
Percentage of assets measured on recurring basis to assets		1.80%	0.30%
Financial instruments held for investment activities		$ 8,870,000	$ 3,114,000
Investment funds valued at net asset value		$ 0	$ 0