Consolidation - Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Total cash	$ 128,847	$ 203,304	$ 173,764	$ 271,545
Total investments	55,225	63,448
Total assets	826,763	698,773
Total liabilities	237,538	112,350
Redeemable noncontrolling interest	49,895	23,071
Equity attributable to stockholders of the Company	539,626	563,542
Non-redeemable noncontrolling interest	(296)	(190)
Total liabilities and equity	826,763	698,773
Balance Before Consolidation of Investment Products [Member]
Condensed Financial Statements, Captions [Line Items]
Total cash	126,470	202,847
Total investments	55,225	63,448
All other assets	17,241	16,060
Consolidated Sponsored Investment Products [Member]
Condensed Financial Statements, Captions [Line Items]
Total cash	16,732	8,687
Total investments	311,429	236,652
All other assets	10,494	6,960
Total liabilities	29,849	12,556
Redeemable noncontrolling interest	49,895	23,071
Consolidated Investment Product [Member]
Condensed Financial Statements, Captions [Line Items]
Total investments	131,862
All other assets	20
Balances as Reported in Condensed Consolidated Balance Sheet [Member]
Condensed Financial Statements, Captions [Line Items]
Total cash	143,322	211,534
Total investments	498,516	300,100
All other assets	184,925	187,139
Total assets	826,763	698,773
Total liabilities	237,538	112,350
Redeemable noncontrolling interest	49,895	23,071
Equity attributable to stockholders of the Company	539,626	563,542
Non-redeemable noncontrolling interest	(296)	(190)
Total liabilities and equity	826,763	698,773
Reportable Legal Entities [Member] | Balance Before Consolidation of Investment Products [Member]
Condensed Financial Statements, Captions [Line Items]
Total cash	126,470	202,847
Total investments	333,307	279,863
All other assets	174,563	180,436
Total assets	634,340	663,146
Total liabilities	95,010	99,794
Redeemable noncontrolling interest	0	0
Equity attributable to stockholders of the Company	539,626	563,542
Non-redeemable noncontrolling interest	(296)	(190)
Total liabilities and equity	634,340	663,146
Reportable Legal Entities [Member] | Consolidated Sponsored Investment Products [Member]
Condensed Financial Statements, Captions [Line Items]
Total cash	16,732	8,687
Total investments	311,429	236,652
All other assets	10,494	6,960
Total assets	338,655	252,299
Total liabilities	29,999	12,813
Redeemable noncontrolling interest	0	0
Equity attributable to stockholders of the Company	308,656	239,486
Non-redeemable noncontrolling interest	0	0
Total liabilities and equity	338,655	252,299
Reportable Legal Entities [Member] | Consolidated Investment Product [Member]
Condensed Financial Statements, Captions [Line Items]
Total cash	120	0
Total investments	131,862	0
All other assets	20	0
Total assets	132,002	0
Total liabilities	112,681	0
Redeemable noncontrolling interest	0	0
Equity attributable to stockholders of the Company	19,321	0
Non-redeemable noncontrolling interest	0	0
Total liabilities and equity	132,002	0
Eliminations and Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Total cash	0	0
Total investments	(278,082)	(216,415)
All other assets	(152)	(257)
Total assets	(278,234)	(216,672)
Total liabilities	(152)	(257)
Redeemable noncontrolling interest	49,895	23,071
Equity attributable to stockholders of the Company	(327,977)	(239,486)
Non-redeemable noncontrolling interest	0	0
Total liabilities and equity	$ (278,234)	$ (216,672)