Consolidated Sponsored Investment Products - Consolidated Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Income Statements, Captions [Line Items]
Total operating revenues	$ 112,137	$ 117,841	$ 112,749	$ 107,871	$ 106,498	$ 100,409	$ 96,140	$ 86,168	$ 450,598	$ 389,215	$ 280,086
Total operating expenses									319,878	275,711	219,641
Operating income (loss)	36,665	38,927	22,502	32,626	33,892	31,630	26,882	21,100	130,720	113,504	60,445
Total other non-operating income (expense)									5,594	8,404	4,358
Income Before Income Taxes									136,314	121,908	64,803
Income tax expense									39,349	44,778	27,030
Net income (loss)									96,965	77,130	37,773
Noncontrolling interests									735	(1,940)	(101)
Allocation of earnings to preferred stockholders									0		(64)
Net income (loss) attributable to the Company	18,879	37,340	19,543	21,938	24,756	21,089	15,385	13,960	97,700	75,190	37,608
Reportable Legal Entities [Member] | Balance Before Consolidation of Investment Products [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues									451,259	389,202	279,919
Total operating expenses									316,840	274,913	219,326
Operating income (loss)									134,419	114,289	60,593
Total other non-operating income (expense)									2,502	5,620	4,106
Income Before Income Taxes									136,921	119,909	64,699
Income tax expense									39,349	44,778	27,030
Net income (loss)									97,572	75,131	37,669
Noncontrolling interests									128	59	3
Allocation of earnings to preferred stockholders											(64)
Net income (loss) attributable to the Company									97,700	75,190	37,608
Reportable Legal Entities [Member] | Consolidated Sponsored Investment Products [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues									0
Total operating expenses									3,699	785	148
Operating income (loss)									(3,699)	(785)	(148)
Total other non-operating income (expense)									2,619	6,098	2,649
Income Before Income Taxes									(1,080)	5,313	2,501
Income tax expense									0
Net income (loss)									(1,080)	5,313	2,501
Noncontrolling interests									0
Net income (loss) attributable to the Company									(1,080)	5,313	2,501
Eliminations and Adjustments [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues									(661)	13	167
Total operating expenses									(661)	13	167
Operating income (loss)									0
Total other non-operating income (expense)									473	(3,314)	(2,397)
Income Before Income Taxes									473	(3,314)	(2,397)
Income tax expense									0
Net income (loss)									473	(3,314)	(2,397)
Noncontrolling interests									607	(1,999)	(104)
Net income (loss) attributable to the Company									$ 1,080	$ (5,313)	$ (2,501)