Equity - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended			1 Months Ended	0 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 30, 2010	Apr. 02, 2010	Apr. 02, 2013
Stockholders Equity Note [Line Items]
Dividends declared per common share (usd per share)	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 1.80	$ 1.80	$ 1.80
Dividend equivalents paid on outstanding restricted stock units									$ 5.5	$ 6.2	$ 5.6
Restricted Stock Units
Stockholders Equity Note [Line Items]
Restricted stock units vested and issued as common stock (shares)									1,019,989	693,691
Repurchased shares for award (shares)									404,611	226,505
Average repurchase price of shares for award (usd per share)									$ 51.26	$ 56.83
Common Stock
Stockholders Equity Note [Line Items]
Repurchased common stock (shares)									334,935	853,870
Average repurchase price of common stock (usd per share)									$ 46.09	$ 49.81
Contingent Convertible Preferred Stock | Greenhill & Co. Australia Pty Limited
Stockholders Equity Note [Line Items]
Shares issued in acquisition (shares)												1,099,877
Contingent Convertible Preferred Stock | Tranche One - Performance Stock | Greenhill & Co. Australia Pty Limited
Stockholders Equity Note [Line Items]
Convertible Preferred Stock, Shares Issued upon Conversion														659,926
Conversion of Stock, Amount Converted													$ 32.5
Contingent Convertible Preferred Stock | Tranche Two - Performance Stock | Greenhill & Co. Australia Pty Limited
Stockholders Equity Note [Line Items]
Conversion to common stock shares if revenue targets are achieved (shares)														439,951