Consolidated Sponsored Investment Products - Condensed Consolidated Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Condensed Income Statements, Captions [Line Items]
Total operating revenues	$ 117,841	$ 100,409	$ 338,461	$ 282,717
Total operating expenses	78,914	68,779	244,406	203,105
Operating income (loss)	38,927	31,630	94,055	79,612
Total other non-operating income (expense)	(3,737)	2,239	8,810	993
Income Before Income Taxes	35,190	33,869	102,865	80,605
Income tax (benefit) expense	(1,805)	12,567	24,311	30,335
Net income (loss)	36,995	21,302	78,554	50,270
Noncontrolling interests	345	(213)	267	164
Net income (loss) attributable to common stockholders	37,340	21,089	78,821	50,434
Reportable Legal Entities [Member] | Balance Before Consolidation of Investment Products [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues	117,808	100,319	338,534	282,575
Total operating expenses	77,668	68,547	242,032	202,557
Operating income (loss)	40,140	31,772	96,502	80,018
Total other non-operating income (expense)	(4,649)	1,870	6,545	705
Income Before Income Taxes	35,491	33,642	103,047	80,723
Income tax (benefit) expense	(1,805)	12,567	24,311	30,335
Net income (loss)	37,296	21,075	78,736	50,388
Noncontrolling interests	44	14	85	46
Net income (loss) attributable to common stockholders	37,340	21,089	78,821	50,434
Reportable Legal Entities [Member] | Consolidated Sponsored Investment Products [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues	0		0
Total operating expenses	1,213	142	2,447	406
Operating income (loss)	(1,213)	(142)	(2,447)	(406)
Total other non-operating income (expense)	(3,109)	923	5,883	(768)
Income Before Income Taxes	(4,322)	781	3,436	(1,174)
Income tax (benefit) expense	0		0
Net income (loss)	(4,322)	781	3,436	(1,174)
Noncontrolling interests	0		0
Net income (loss) attributable to common stockholders	(4,322)	781	3,436	(1,174)
Eliminations and Adjustments [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues	33	90	(73)	142
Total operating expenses	33	90	(73)	142
Operating income (loss)	0		0
Total other non-operating income (expense)	4,021	(554)	(3,618)	1,056
Income Before Income Taxes	4,021	(554)	(3,618)	1,056
Income tax (benefit) expense	0		0
Net income (loss)	4,021	(554)	(3,618)	1,056
Noncontrolling interests	301	(227)	182	118
Net income (loss) attributable to common stockholders	$ 4,322	$ (781)	$ (3,436)	$ 1,174