Fair Value Measurements (Summary Of Changes In Fair Value Of The Company's Level 3 Assets) (Details) (USD $) In Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Beginning Balance	$ 171,616	$ 136,562	$ 163,798	$ 136,219
Net Unrealized/Realized Gains (Losses) Included In Revenue-Other	3,548	1,531	3,827	4,741
Purchases/Acquisitions	922	5,559	13,075	5,560
Sales/Dispositions	(2,052)	(1,190)	(9,355)	(1,193)
Foreign Currency Translation Adjustments	805	(3,754)	3,494	(6,619)
Ending Balance	174,839	138,708	174,839	138,708
Equities [Member]
Beginning Balance	129	298	316	305
Net Unrealized/Realized Gains (Losses) Included In Revenue-Other	3	(4)	3	(12)
Purchases/Acquisitions		5		6
Sales/Dispositions			(195)
Foreign Currency Translation Adjustments	3		11
Ending Balance	135	299	135	299
Private Equity [Member]
Beginning Balance	171,487	136,264	163,482	135,914
Net Unrealized/Realized Gains (Losses) Included In Revenue-Other	3,545	1,535	3,824	4,753
Purchases/Acquisitions	922	5,554	13,075	5,554
Sales/Dispositions	(2,052)	(1,190)	(9,160)	(1,193)
Foreign Currency Translation Adjustments	802	(3,754)	3,483	(6,619)
Ending Balance	$ 174,704	$ 138,409	$ 174,704	$ 138,409

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Foreign Currency Translation Adjustments

Name:	laz_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetForeignCurrencyTranslationAdjustments
Namespace Prefix:	laz
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Purchases/Acquisitions that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ Details

Name:	laz_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesAcquisitions
Namespace Prefix:	laz
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Sales/Dispositions that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ Details

Name:	laz_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSalesDispositions
Namespace Prefix:	laz
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This element represents total gains or losses for the period (realized and unrealized), arising from assets measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or resulted in a change in net asset value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 157
-Paragraph 32
-Subparagraph c(1)

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 157
-Paragraph 32
-Subparagraph c

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant