Consolidation - Condensed Consolidated Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Condensed Income Statements, Captions [Line Items]
Total operating revenues	$ 80,085	$ 99,656	$ 160,380	$ 203,487
Total operating expenses	71,342	83,448	138,887	162,737
Operating Income	8,743	16,208	21,493	40,750
Total other non-operating income, net	6,044	627	12,720	6,678
Income Before Income Taxes	14,787	16,835	34,213	47,428
Income taxes	6,087	7,823	13,643	18,691
Net Income	8,700	9,012	20,570	28,737
Noncontrolling interests	(612)	765	(119)	382
Net Income Attributable to Common Stockholders	8,088	9,777	20,451	29,119
Reportable Legal Entities [Member] | Balance Before Consolidation of Investment Products [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues	80,058	100,052	160,562	204,284
Total operating expenses	66,723	82,491	133,080	160,962
Operating Income	13,335	17,561	27,482	43,322
Total other non-operating income, net	840	(127)	6,612	4,288
Income Before Income Taxes	14,175	17,434	34,094	47,610
Income taxes	6,087	7,823	13,643	18,691
Net Income	8,088	9,611	20,451	28,919
Noncontrolling interests	0	166	0	200
Net Income Attributable to Common Stockholders	8,088	9,777	20,451	29,119
Reportable Legal Entities [Member] | Consolidated Investment Product [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues	0	0	0	0
Total operating expenses	3,947	0	164	2,492
Operating Income	(3,947)	0	(164)	(2,492)
Total other non-operating income, net	3,947	0	1,370	4,407
Income Before Income Taxes	0	0	1,206	1,915
Income taxes	0	0	0	0
Net Income	0	0	1,206	1,915
Noncontrolling interests	0	0	0	0
Net Income Attributable to Common Stockholders	0	0	1,206	1,915
Eliminations and Adjustments [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues	27	(396)	(182)	(797)
Total operating expenses	27	(396)	(182)	(797)
Operating Income	0	0	0	0
Total other non-operating income, net	(3,535)	898	(8,448)	(2,380)
Income Before Income Taxes	(3,535)	898	(8,448)	(2,380)
Income taxes	0	0	0	0
Net Income	(3,535)	898	(8,448)	(2,380)
Noncontrolling interests	(612)	599	(119)	182
Net Income Attributable to Common Stockholders	(4,147)	1,497	(8,567)	(2,198)
Consolidated Sponsored Investment Products - VOEs [Member] | Reportable Legal Entities [Member] | Consolidated Sponsored Investment Products [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues	0	0	0	0
Total operating expenses	556	43	4,003	0
Operating Income	(556)	(43)	(4,003)	0
Total other non-operating income, net	3,947	101	6,508	0
Income Before Income Taxes	3,391	58	2,505	0
Income taxes	0	0	0	0
Net Income	3,391	58	2,505	0
Noncontrolling interests	0	0	0	0
Net Income Attributable to Common Stockholders	3,391	58	2,505	0
Consolidated Sponsored Investment Product - VIE [Member] | Reportable Legal Entities [Member] | Consolidated Sponsored Investment Products [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues	0	0	0	0
Total operating expenses	89	1,310	1,822	80
Operating Income	(89)	(1,310)	(1,822)	(80)
Total other non-operating income, net	845	(245)	6,678	363
Income Before Income Taxes	756	(1,555)	4,856	283
Income taxes	0	0	0	0
Net Income	756	(1,555)	4,856	283
Noncontrolling interests	0	0	0	0
Net Income Attributable to Common Stockholders	$ 756	$ (1,555)	$ 4,856	$ 283