Equity - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended								0 Months Ended			0 Months Ended	12 Months Ended			3 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Restricted Stock Units	Dec. 31, 2012 Restricted Stock Units	Dec. 31, 2011 Restricted Stock Units	Dec. 31, 2013 Common Stock	Dec. 31, 2012 Common Stock	Apr. 02, 2010 Contingent Convertible Preferred Stock Greenhill & Co. Australia Pty Limited	Apr. 02, 2010 Tranche One - Performance Stock Contingent Convertible Preferred Stock Greenhill & Co. Australia Pty Limited	Apr. 03, 2013 Tranche One - Performance Stock Contingent Convertible Preferred Stock Greenhill & Co. Australia Pty Limited	Apr. 02, 2013 Tranche One - Performance RSU's [Member] Performance Based Restricted Units [Member] Greenhill & Co. Australia Pty Limited	Dec. 31, 2013 Tranche One - Performance RSU's [Member] Performance Based Restricted Units [Member] Greenhill & Co. Australia Pty Limited	Dec. 31, 2012 Tranche One - Performance RSU's [Member] Performance Based Restricted Units [Member] Greenhill & Co. Australia Pty Limited	Dec. 31, 2011 Tranche One - Performance RSU's [Member] Performance Based Restricted Units [Member] Greenhill & Co. Australia Pty Limited	Dec. 31, 2013 Tranche Two - Performance RSU's [Member] Performance Based Restricted Units [Member] Greenhill & Co. Australia Pty Limited	Dec. 31, 2013 Tranche Two - Performance Stock Contingent Convertible Preferred Stock Greenhill & Co. Australia Pty Limited
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									$ 6.2	$ 5.6	$ 4.8
Restricted stock units vested and issued as common stock (shares)												693,691	654,612											41,794
Repurchased shares for award (shares)												226,505	181,820
Average repurchase price of shares for award (usd per share)												$ 56.83	$ 45.29
Repurchased common stock (shares)															853,870	1,714,614
Average repurchase price of common stock (usd per share)															$ 49.81	$ 43.70
Shares issued in acquisition (shares)																	1,099,877
Convertible Preferred Stock, Shares Issued upon Conversion																			659,926
Conversion to common stock shares if revenue targets are achieved (shares)																									439,951
Conversion of Stock, Amount Converted																		32.5
Stock Issued During Period, Shares, Restricted Stock Award, Gross																				62,690
Compensation expense from the vesting of restricted stock units												$ 56.1	$ 54.2	$ 52.8							$ 0.4	$ 1.7	$ 3.0