Consolidation - Condensed Consolidated Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Income Statements, Captions [Line Items]
Total operating revenues	$ 86,115	$ 92,375	$ 99,656	$ 103,831	$ 112,137	$ 117,841	$ 112,749	$ 107,871	$ 381,977	$ 450,598	$ 389,215
Total operating expenses									301,599	319,878	275,711
Operating Income	16,506	23,122	16,208	24,542	36,665	38,927	22,502	32,626	80,378	130,720	113,504
Total other non-operating expense									(12,735)	5,594	8,404
Income Before Income Taxes									67,643	136,314	121,908
Income tax expense									36,972	39,349	44,778
Net Income									30,671	96,965	77,130
Noncontrolling interests									4,435	735	(1,940)
Net Income Attributable to Common Stockholders	$ 6,636	$ (649)	$ 9,777	$ 19,342	$ 18,879	$ 37,340	$ 19,543	$ 21,938	35,106	97,700	75,190
Reportable Legal Entities [Member] | Balance Before Consolidation of Investment Products [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues									383,581	451,259	389,202
Total operating expenses									297,465	316,840	274,913
Operating Income									86,116	134,419	114,289
Total other non-operating expense									(14,214)	2,502	5,620
Income Before Income Taxes									71,902	136,921	119,909
Income tax expense									36,972	39,349	44,778
Net Income									34,930	97,572	75,131
Noncontrolling interests									176	128	59
Net Income Attributable to Common Stockholders									35,106	97,700	75,190
Reportable Legal Entities [Member] | Consolidated Sponsored Investment Products [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues									0	0	0
Total operating expenses									5,738	3,699	785
Operating Income									(5,738)	(3,699)	(785)
Total other non-operating expense									(11,677)	2,619	6,098
Income Before Income Taxes									(17,415)	(1,080)	5,313
Income tax expense									0	0	0
Net Income									(17,415)	(1,080)	5,313
Noncontrolling interests									0	0	0
Net Income Attributable to Common Stockholders									(17,415)	(1,080)	5,313
Reportable Legal Entities [Member] | Consolidated Investment Product [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues									0	0	0
Total operating expenses									0	0	0
Operating Income									0	0	0
Total other non-operating expense									(1,832)	0	0
Income Before Income Taxes									(1,832)	0	0
Income tax expense									0	0	0
Net Income									(1,832)	0	0
Noncontrolling interests									0	0	0
Net Income Attributable to Common Stockholders									(1,832)	0	0
Eliminations and Adjustments [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues									(1,604)	(661)	13
Total operating expenses									(1,604)	(661)	13
Operating Income									0	0	0
Total other non-operating expense									14,988	473	(3,314)
Income Before Income Taxes									14,988	473	(3,314)
Income tax expense									0	0	0
Net Income									14,988	473	(3,314)
Noncontrolling interests									4,259	607	(1,999)
Net Income Attributable to Common Stockholders									$ 19,247	$ 1,080	$ (5,313)