Consolidation - Condensed Consolidated Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Condensed Income Statements, Captions [Line Items]
Total operating revenues	$ 80,295	$ 103,831
Total operating expenses	67,545	79,289
Operating Income	12,750	24,542
Income Before Income Taxes	19,426	30,593
Income taxes	7,556	10,868
Net Income	11,870	19,725
Noncontrolling interests	493	(383)
Net Income Attributable to Common Stockholders	12,363	19,342
Consolidated Entities [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues	80,295	103,831
Total operating expenses	67,545	79,289
Operating Income	12,750	24,542
Total other non-operating income, net	6,676	6,051
Income Before Income Taxes	19,426	30,593
Income taxes	7,556	10,868
Net Income	11,870	19,725
Noncontrolling interests	493	(383)
Net Income Attributable to Common Stockholders	12,363	19,342
Reportable Legal Entities [Member] | Balance Before Consolidation of Investment Products [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues	80,504	104,232
Total operating expenses	66,356	78,471
Operating Income	14,148	25,761
Total other non-operating income, net	5,771	4,415
Income Before Income Taxes	19,919	30,176
Income taxes	7,556	10,868
Net Income	12,363	19,308
Noncontrolling interests	0	34
Net Income Attributable to Common Stockholders	12,363	19,342
Reportable Legal Entities [Member] | Consolidated Investment Product [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues	0	0
Total operating expenses	56	0
Operating Income	(56)	0
Total other non-operating income, net	2,561	0
Income Before Income Taxes	2,505	0
Income taxes	0	0
Net Income	2,505	0
Noncontrolling interests	0	0
Net Income Attributable to Common Stockholders	2,505	0
Eliminations and Adjustments [Member]
Condensed Income Statements, Captions [Line Items]
Total operating revenues	(209)	(401)
Total operating expenses	(209)	(401)
Operating Income	0	0
Total other non-operating income, net	(4,913)	(3,278)
Income Before Income Taxes	(4,913)	(3,278)
Income taxes	0	0
Net Income	(4,913)	(3,278)
Noncontrolling interests	493	(417)
Net Income Attributable to Common Stockholders	(4,420)	(3,695)
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
Total operating expenses	1,267	1,182
Operating Income	(1,267)	(1,182)
Total other non-operating income, net	2,732	4,653
Income Before Income Taxes	1,465	3,471
Income taxes	0	0
Net Income	1,465	3,471
Noncontrolling interests	0	0
Net Income Attributable to Common Stockholders	1,465	3,471
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
Total operating expenses	75	37
Operating Income	(75)	(37)
Total other non-operating income, net	525	261
Income Before Income Taxes	450	224
Income taxes	0	0
Net Income	450	224
Noncontrolling interests	0	0
Net Income Attributable to Common Stockholders	$ 450	$ 224