Selected Quarterly Data (Unaudited) (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
Revenue	$ 139,917	$ 108,413	$ 114,489	$ 106,345	$ 118,980	$ 92,028	$ 87,886	$ 85,046	$ 469,164	$ 383,940	$ 365,546
Reimbursable expenses	5,218	3,299	4,422	2,598	5,573	2,395	3,074	1,892	15,537	12,934	9,485
Total revenue	145,135	111,712	118,911	108,943	124,553	94,423	90,960	86,938	484,701	396,874	375,031
Operating income	13,993	10,340	13,318	9,741	18,312	9,117	8,267	9,577	47,392	45,273	43,222
Net income	6,655	6,216	7,739	5,691	11,155	6,436	5,647	6,491	26,301	29,729	29,346
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
Common Stock Class A [Member]
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 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
Common Stock Class B [Member]
Weighted average shares of Class A common stock outstanding, basic		2,897	3,961	9,050	10,650	10,813	10,947	11,130		10,883
Weighted average shares of Class A common stock outstanding, diluted	2,002	2,897	3,961	9,050	10,650	10,813	10,947	11,130	4,466	10,883