Investments - Investment Revenues (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Principal Transaction Revenue [Line Items]
Net realized and unrealized gains (losses) on investments in merchant banking funds	$ (240)us-gaap_GainLossOnInvestments	$ (5,231)us-gaap_GainLossOnInvestments
Interest income	229us-gaap_InterestIncomeOperating	270us-gaap_InterestIncomeOperating
Total investment revenues (losses)	(9)ghl_InvestmentRevenues	(4,886)ghl_InvestmentRevenues
Merchant Banking Funds
Principal Transaction Revenue [Line Items]
Net realized and unrealized gains (losses) on investments in merchant banking funds	(238)	(5,205)
Deferred gain on sale of certain merchant banking assets	$ 0	$ 49

- Definition

This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

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