Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended								12 Months Ended
	Apr. 01, 2015	Apr. 01, 2013	Apr. 30, 2010	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 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												$ 6.5	$ 5.5	$ 6.2
Cogent Partners, LP
Stockholders Equity Note [Line Items]
Common stock paid at acquisition closing (shares)	779,454
Possible contingent consideration (shares)	334,048
Fair value of contingent common shares	$ 11.9
Restricted Stock Units
Stockholders Equity Note [Line Items]
Restricted stock units vested and issued as common stock (shares)												826,673	1,019,989
Repurchased shares for award (shares)												341,235	404,611
Average repurchase price of shares for award (usd per share)												$ 34.88	$ 51.26
Common Stock
Stockholders Equity Note [Line Items]
Repurchased common stock (shares)													334,935
Average repurchase price of common stock (usd per share)													$ 46.09
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]
Conversion of performance stock to common stock (shares)		659,926
Fair value of performance stock		$ 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
Performance stock fair value at acquisition date	$ 14.4