Fair Value Measurements - Fair Value of Certain Investments Based on Nav (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value	179,417	187,212
Unfunded Commitments	31,482	52,197
Alternative investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value	57,890	59,622
Alternative investments [Member] \| Daily [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Frequency	daily (10%)	daily (17%)
Alternative investments [Member] \| Weekly [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Frequency	weekly (9%)	weekly (8%)
Alternative investments [Member] \| Monthly [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Frequency	monthly (38%)	monthly (37%)
Alternative investments [Member] \| Quarterly [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Frequency	quarterly (43%)	quarterly (38%)
Alternative investments [Member] \| Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Notice Period	30 days	30 days
Alternative investments [Member] \| Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Notice Period	120 days	120 days
Debt funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Notice Period	30 days	30 days
Fair value	4	267
Debt funds [Member] \| Daily [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Frequency	daily (100%)	daily (100%)
Equity funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Notice Period		30 days
Fair value	9,079	4,605
% of Fair Value Not Redeemable	2.00%	5.00%
Estimated Liquidation Period of Investments Not Redeemable, % Next 5 Years		5.00%
Estimated Liquidation Period of Investments Not Redeemable, % Thereafter	2.00%
Equity funds [Member] \| Daily [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Frequency	daily (37%)	daily (95%)
Equity funds [Member] \| Monthly [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Frequency	monthly (61%)
Equity funds [Member] \| Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Notice Period	30 days
Equity funds [Member] \| Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments Redeemable, Redemption Notice Period	120 days
Private equity [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value	112,444	122,718
Unfunded Commitments	31,482	52,197
% of Fair Value Not Redeemable	100.00%	100.00%
Estimated Liquidation Period of Investments Not Redeemable, % Next 5 Years	13.00%	33.00%
Estimated Liquidation Period of Investments Not Redeemable, % 5-10 Years	39.00%	28.00%
Estimated Liquidation Period of Investments Not Redeemable, % Thereafter	48.00%	39.00%

Percentage of estimated liquidation period of investments not redeemable, five to ten years.

No definition available.

Name:	laz_PercentageOfEstimatedLiquidationPeriodOfInvestmentsNotRedeemableFiveToTenYears
Namespace Prefix:	laz_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of estimated liquidation period of investments not redeemable, next five years.

+ References

No definition available.

+ Details

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Namespace Prefix:	laz_
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- Definition

Percentage of estimated liquidation period of investments not redeemable, thereafter.

+ References

No definition available.

+ Details

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Namespace Prefix:	laz_
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Balance Type:	na
Period Type:	duration

- Definition

Percentage of fair value not redeemable.

+ References

No definition available.

+ Details

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Namespace Prefix:	laz_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the total fair market value of investments in certain entities that calculate net asset value per share (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments). This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements, but may also be used in both the balance sheet and disclosure in the same submission. This item represents alternative investments as of the balance sheet date which may include investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds, and any other investment in which investee management calculates net asset value per share as the primary metric for investment value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 6A

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=SL6742756-110258

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

States the frequency (for example, annually, semi-annually, quarterly) with which the entity can redeem its investment, or portion thereof, for each of those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments), by major category.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 6A

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=SL6742756-110258

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

States the notice requirement (for example, 30 to 60 days) the entity is required to deliver before it can redeem its investment, or portion thereof, for each of those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments), by major category.

+ References

+ Details

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Data Type:	xbrli:stringItemType
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- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 6A

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=SL6742756-110258

+ Details

Name:	us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant