Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended								12 Months Ended
	Apr. 01, 2015	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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										$ 8.5	$ 6.5	$ 5.5
Common Stock
Stockholders Equity Note [Line Items]
Repurchased common stock (shares)										891,017
Average repurchase price of common stock (usd per share)										$ 22.63
Restricted stock units
Stockholders Equity Note [Line Items]
Restricted stock units vested and issued as common stock (shares)										852,218	826,673
Repurchased shares for award (shares)										319,573	341,235
Average repurchase price of shares for award (usd per share)										$ 25.10	$ 34.88
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
Greenhill & Co. Australia Pty Limited | Contingent Convertible Preferred Stock
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