Summary Selected Quarterly Financial Data - Revised Consolidated Statements of Income (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								9 Months Ended	12 Months Ended			3 Months Ended							6 Months Ended		9 Months Ended		12 Months Ended		3 Months Ended							6 Months Ended		9 Months Ended		12 Months Ended		3 Months Ended							6 Months Ended		9 Months Ended		12 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	Sep. 30, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2013 As Previously Reported [Member]	Jun. 30, 2013 As Previously Reported [Member]	Mar. 31, 2013 As Previously Reported [Member]	Dec. 31, 2012 As Previously Reported [Member]	Sep. 30, 2012 As Previously Reported [Member]	Jun. 30, 2012 As Previously Reported [Member]	Mar. 31, 2012 As Previously Reported [Member]	Jun. 30, 2013 As Previously Reported [Member]	Jun. 30, 2012 As Previously Reported [Member]	Sep. 30, 2013 As Previously Reported [Member]	Sep. 30, 2012 As Previously Reported [Member]	Dec. 31, 2012 As Previously Reported [Member]	Dec. 31, 2011 As Previously Reported [Member]	Sep. 30, 2013 Adjustments [Member]	Jun. 30, 2013 Adjustments [Member]	Mar. 31, 2013 Adjustments [Member]	Dec. 31, 2012 Adjustments [Member]	Sep. 30, 2012 Adjustments [Member]	Jun. 30, 2012 Adjustments [Member]	Mar. 31, 2012 Adjustments [Member]	Jun. 30, 2013 Adjustments [Member]	Jun. 30, 2012 Adjustments [Member]	Sep. 30, 2013 Adjustments [Member]	Sep. 30, 2012 Adjustments [Member]	Dec. 31, 2012 Adjustments [Member]	Dec. 31, 2011 Adjustments [Member]	Sep. 30, 2013 As Revised [Member]	Jun. 30, 2013 As Revised [Member]	Mar. 31, 2013 As Revised [Member]	Dec. 31, 2012 As Revised [Member]	Sep. 30, 2012 As Revised [Member]	Jun. 30, 2012 As Revised [Member]	Mar. 31, 2012 As Revised [Member]	Jun. 30, 2013 As Revised [Member]	Jun. 30, 2012 As Revised [Member]	Sep. 30, 2013 As Revised [Member]	Sep. 30, 2012 As Revised [Member]	Dec. 31, 2012 As Revised [Member]	Dec. 31, 2011 As Revised [Member]
Revenues:
Research services										$ 202,843	$ 203,091	$ 191,495		$ 50,512	$ 50,378	$ 51,866	$ 50,300	$ 51,072	$ 49,760	$ 100,890	$ 100,832		$ 151,132	$ 202,998	$ 191,648		$ 800	$ (100)	$ 80	$ 209	$ (100)	$ (96)	$ 700	$ (196)		$ 13	$ 93	$ (153)		$ 51,312	$ 50,278	$ 51,946	$ 50,509	$ 50,972	$ 49,664	$ 101,590	$ 100,636		$ 151,145	$ 203,091	$ 191,495
Advisory services and other										94,807	89,940	91,840		27,652	21,121	23,200	18,211	28,021	20,500	48,773	48,521		66,732	89,932	91,968		(11)	(38)	15	53	(61)	1	(49)	(60)		(7)	8	(128)		27,641	21,083	23,215	18,264	27,960	20,501	48,724	48,461		66,725	89,940	91,840
Total revenues	77,521	69,815	78,953	71,361	75,161	68,773	78,932	70,165		297,650	293,031	283,335		78,164	71,499	75,066	68,511	79,093	70,260	149,663	149,353		217,864	292,930	283,616		789	(138)	95	262	(161)	(95)	651	(256)		6	101	(281)		78,953	71,361	75,161	68,773	78,932	70,165	150,314	149,097		217,870	293,031	283,335
Income from operations	4,464	4,301	9,788	3,280	7,945	7,130	11,152	4,533		21,833	30,760	36,716		8,999	3,418	7,393	7,177	11,461	4,628	12,417	16,089		23,266	30,659	36,997		789	(138)	552	(47)	(309)	(95)	651	(404)		(451)	101	(281)		9,788	3,280	7,945	7,130	11,152	4,533	13,068	15,685		22,815	30,760	36,716
Income before income taxes										19,992	32,154	36,947		9,203	3,743	7,058	7,681	11,675	5,096	12,946	16,771		24,452	31,510	38,645		938	(287)	487	561	(309)	(95)	651	(404)		157	644	(1,698)		10,141	3,456	7,545	8,242	11,366	5,001	13,597	16,367		24,609	32,154	36,947
Income tax provision										6,968	5,858	14,956	1,813	3,581	1,402	2,807	(2,692)	3,906	1,915	4,983	5,821	7,056	3,129	5,936	15,635	(74)	375	(115)	261	(177)	(124)	(38)	260	(162)	(74)	(339)	(78)	(679)	1,739	3,956	1,287	3,068	(2,869)	3,782	1,877	5,243	5,659	6,982	2,790	5,858	14,956
Net income	2,161	2,509	6,185	2,169	4,477	11,111	7,584	3,124	10,863	13,024	26,296	21,991	2,435	5,622	2,341	4,251	10,373	7,769	3,181	7,963	10,950	10,789	21,323	25,574	23,010	74	563	(172)	226	738	(185)	(57)	391	(242)	74	496	722	(1,019)	2,509	6,185	2,169	4,477	11,111	7,584	3,124	8,354	10,708	10,863	21,819	26,296	21,991
Basic income per common share	$ 0.11	$ 0.12	$ 0.29	$ 0.10	$ 0.20	$ 0.50	$ 0.34	$ 0.14		$ 0.62	$ 1.17	$ 0.97	$ 0.12	$ 0.26	$ 0.10	$ 0.19	$ 0.46	$ 0.34	$ 0.14	$ 0.37	$ 0.48	$ 0.51	$ 0.94	$ 1.14	$ 1.02		$ 0.03		$ 0.01	$ 0.04			$ 0.01	$ (0.01)		$ 0.03	$ 0.03	$ (0.05)	$ 0.12	$ 0.29	$ 0.10	$ 0.20	$ 0.05	$ 0.34	$ 0.14	$ 0.38	$ 0.47	$ 0.51	$ 0.97	$ 1.17	$ 0.97
Diluted income per common share	$ 0.11	$ 0.12	$ 0.28	$ 0.10	$ 0.20	$ 0.49	$ 0.33	$ 0.13		$ 0.61	$ 1.15	$ 0.95	$ 0.12	$ 0.26	$ 0.10	$ 0.19	$ 0.45	$ 0.34	$ 0.14	$ 0.36	$ 0.47	$ 0.50	$ 0.93	$ 1.12	$ 0.99		$ 0.03		$ 0.01	$ 0.04	$ (0.01)	$ (0.01)	$ 0.02	$ (0.01)		$ 0.02	$ 0.03	$ (0.04)	$ 0.12	$ 0.28	$ 0.10	$ 0.20	$ 0.49	$ 0.33	$ 0.13	$ 0.38	$ 0.46	$ 0.50	$ 0.95	$ 1.15	$ 0.95
General and administrative										38,280	36,866	33,284			9,487	10,199	8,411	8,645			18,256		26,667						(457)	309	148			148		457					9,487	9,742	8,720	8,793			18,404		27,124
Total operating expenses										275,817	262,271	246,619			68,081	67,673	61,334	67,632			133,264		194,598						(457)	309	148			148		457					68,081	67,216	61,643	67,780			133,412		195,055
Income from operations	4,464	4,301	9,788	3,280	7,945	7,130	11,152	4,533		21,833	30,760	36,716		8,999	3,418	7,393	7,177	11,461	4,628	12,417	16,089		23,266	30,659	36,997		789	(138)	552	(47)	(309)	(95)	651	(404)		(451)	101	(281)		9,788	3,280	7,945	7,130	11,152	4,533	13,068	15,685		22,815	30,760	36,716
Gains (losses) on investments, net		600	100	(100)	(100)					(2,433)	94	(399)		(51)	(51)	(739)	147			(102)			290	(449)	1,018		149	(149)	(65)	608						608	543	(1,417)		98	(200)	(804)	755			(102)			898	94	(399)
Income before income taxes										19,992	32,154	36,947		9,203	3,743	7,058	7,681	11,675	5,096	12,946	16,771		24,452	31,510	38,645		938	(287)	487	561	(309)	(95)	651	(404)		157	644	(1,698)		10,141	3,456	7,545	8,242	11,366	5,001	13,597	16,367		24,609	32,154	36,947
Income tax provision										6,968	5,858	14,956	1,813	3,581	1,402	2,807	(2,692)	3,906	1,915	4,983	5,821	7,056	3,129	5,936	15,635	(74)	375	(115)	261	(177)	(124)	(38)	260	(162)	(74)	(339)	(78)	(679)	1,739	3,956	1,287	3,068	(2,869)	3,782	1,877	5,243	5,659	6,982	2,790	5,858	14,956
Net income	$ 2,161	$ 2,509	$ 6,185	$ 2,169	$ 4,477	$ 11,111	$ 7,584	$ 3,124	$ 10,863	$ 13,024	$ 26,296	$ 21,991	$ 2,435	$ 5,622	$ 2,341	$ 4,251	$ 10,373	$ 7,769	$ 3,181	$ 7,963	$ 10,950	$ 10,789	$ 21,323	$ 25,574	$ 23,010	$ 74	$ 563	$ (172)	$ 226	$ 738	$ (185)	$ (57)	$ 391	$ (242)	$ 74	$ 496	$ 722	$ (1,019)	$ 2,509	$ 6,185	$ 2,169	$ 4,477	$ 11,111	$ 7,584	$ 3,124	$ 8,354	$ 10,708	$ 10,863	$ 21,819	$ 26,296	$ 21,991
Basic income per common share	$ 0.11	$ 0.12	$ 0.29	$ 0.10	$ 0.20	$ 0.50	$ 0.34	$ 0.14		$ 0.62	$ 1.17	$ 0.97	$ 0.12	$ 0.26	$ 0.10	$ 0.19	$ 0.46	$ 0.34	$ 0.14	$ 0.37	$ 0.48	$ 0.51	$ 0.94	$ 1.14	$ 1.02		$ 0.03		$ 0.01	$ 0.04			$ 0.01	$ (0.01)		$ 0.03	$ 0.03	$ (0.05)	$ 0.12	$ 0.29	$ 0.10	$ 0.20	$ 0.05	$ 0.34	$ 0.14	$ 0.38	$ 0.47	$ 0.51	$ 0.97	$ 1.17	$ 0.97
Diluted income per common share	$ 0.11	$ 0.12	$ 0.28	$ 0.10	$ 0.20	$ 0.49	$ 0.33	$ 0.13		$ 0.61	$ 1.15	$ 0.95	$ 0.12	$ 0.26	$ 0.10	$ 0.19	$ 0.45	$ 0.34	$ 0.14	$ 0.36	$ 0.47	$ 0.50	$ 0.93	$ 1.12	$ 0.99		$ 0.03		$ 0.01	$ 0.04	$ (0.01)	$ (0.01)	$ 0.02	$ (0.01)		$ 0.02	$ 0.03	$ (0.04)	$ 0.12	$ 0.28	$ 0.10	$ 0.20	$ 0.49	$ 0.33	$ 0.13	$ 0.38	$ 0.46	$ 0.50	$ 0.95	$ 1.15	$ 0.95