Earnings per Share Earnings Per Share Two Class Method (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income attributable to Duff & Phelps Corporation	$ 6,065	$ 5,701	$ 6,593	$ 3,905	$ 7,045	$ 4,032	$ 3,424	$ 4,113	$ 22,264	$ 18,614	$ 16,765
Undistributed Earnings Allocated to Participating Securities									(992)	(1,100)	(1,132)
Earnings Available for Common Stock									$ 21,272	$ 17,514	$ 15,633
Common Stock Class A [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted average shares of Class A common stock outstanding, basic	35,704	34,830	33,788	28,702	26,685	26,945	27,296	26,910	33,267	26,958	25,170
Weighted Average Number Diluted Shares Outstanding Adjustment									1,318	874	919
Weighted average shares of Class A common stock outstanding, diluted	37,245	36,208	35,076	30,077	27,674	27,060	28,067	27,615	34,585	27,832	26,089
Earnings per share, basic	$ 0.16	$ 0.16	$ 0.18	$ 0.13	$ 0.24	$ 0.14	$ 0.12	$ 0.15	$ 0.64	$ 0.65	$ 0.62
Earnings per share, diluted	$ 0.16	$ 0.15	$ 0.18	$ 0.13	$ 0.23	$ 0.14	$ 0.12	$ 0.14	$ 0.62	$ 0.63	$ 0.60