Consolidation of Sponsored Investment Products - Consolidated Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Condensed Consolidating Financial Information [Line Items]
Total operating revenues	$ 100,409	$ 71,951	$ 282,717	$ 201,167
Total operating expenses	68,779	54,184	203,105	161,194
Operating income (loss)	31,630	17,767	79,612	39,973
Total other non-operating income (expense)	2,239	1,154	993	1,966
Income Before Income Taxes	33,869	18,921	80,605	41,939
Income tax expense	12,567	7,279	30,335	16,480
Net income (loss)	21,302	11,642	50,270	25,459
Noncontrolling interests	(213)		164
Allocation of earnings to preferred stockholders				(64)
Net Income Attributable to Common Stockholders	21,089	11,642	50,434	25,395
Reportable Legal Entities [Member] | Balance Before Consolidation of Investment Products [Member]
Condensed Consolidating Financial Information [Line Items]
Total operating revenues	100,319	71,825	282,575	201,041
Total operating expenses	68,547	54,030	202,557	161,040
Operating income (loss)	31,772	17,795	80,018	40,001
Total other non-operating income (expense)	1,870	1,126	705	1,938
Income Before Income Taxes	33,642	18,921	80,723	41,939
Income tax expense	12,567	7,279	30,335	16,480
Net income (loss)	21,075	11,642	50,388	25,459
Noncontrolling interests	14		46
Allocation of earnings to preferred stockholders				(64)
Net Income Attributable to Common Stockholders	21,089	11,642	50,434	25,395
Reportable Legal Entities [Member] | Consolidated Investment Products [Member]
Condensed Consolidating Financial Information [Line Items]
Total operating revenues
Total operating expenses	142	28	406	28
Operating income (loss)	(142)	(28)	(406)	(28)
Total other non-operating income (expense)	923	639	(768)	639
Income Before Income Taxes	781	611	(1,174)	611
Income tax expense
Net income (loss)	781	611	(1,174)	611
Noncontrolling interests
Allocation of earnings to preferred stockholders
Net Income Attributable to Common Stockholders	781	611	(1,174)	611
Consolidation, Eliminations [Member]
Condensed Consolidating Financial Information [Line Items]
Total operating revenues	90	126	142	126
Total operating expenses	90	126	142	126
Operating income (loss)
Total other non-operating income (expense)	(554)	(611)	1,056	(611)
Income Before Income Taxes	(554)	(611)	1,056	(611)
Income tax expense
Net income (loss)	(554)	(611)	1,056	(611)
Noncontrolling interests	(227)		118
Allocation of earnings to preferred stockholders
Net Income Attributable to Common Stockholders	$ (781)	$ (611)	$ 1,174	$ (611)