Investments - Investment Revenues (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Principal Transaction Revenue [Line Items]
Net realized and unrealized gains (losses)	$ (6,594)us-gaap_GainLossOnInvestments	$ (1,418)us-gaap_GainLossOnInvestments	$ (8,401)us-gaap_GainLossOnInvestments
Deferred gain on sale of certain merchant banking assets	195	195	260
Interest income	1,154us-gaap_InterestIncomeOperating	1,071us-gaap_InterestIncomeOperating	1,676us-gaap_InterestIncomeOperating
Investment revenues	(5,218)ghl_InvestmentRevenues	244ghl_InvestmentRevenues	(6,466)ghl_InvestmentRevenues
Merchant Banking Funds
Principal Transaction Revenue [Line Items]
Net realized and unrealized gains (losses)	(6,567)	(1,884)	(3,422)
Iridium
Principal Transaction Revenue [Line Items]
Net realized and unrealized gains (losses)	$ 0	$ 862	$ (4,980)

- 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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