Consolidation of Sponsored Investment Products - Consolidated Statement of Operation (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Condensed Consolidating Financial Information [Line Items]
Total operating revenues	$ 78,919	$ 71,951	$ 66,818	$ 62,398	$ 56,172	$ 55,457	$ 48,902	$ 44,121	$ 280,086		$ 204,652	$ 144,556
Total operating expenses									219,641		190,749	135,285
Operating income (loss)	20,472	17,767	14,253	7,953	8,194	(2,193)	3,459	4,443	60,445		13,903	9,271
Total other non-operating income (expense)									4,358
Income Before Income Taxes									64,803		12,992	10,155
Income tax expense									27,030		(132,428)	513
Net income (loss)									37,773		145,420	9,642
Noncontrolling interests									(101)
Allocation of earnings to preferred stockholders									(64)		(24,260)	(1,144)
Net Income Attributable to Common Stockholders	12,213	11,642	8,367	5,386	109,555	(3,483)	2,043	2,979	37,608		111,678	5,209
Before Consolidation [Member]
Condensed Consolidating Financial Information [Line Items]
Total operating revenues									279,919
Total operating expenses									219,326
Operating income (loss)									60,593
Total other non-operating income (expense)									4,106
Income Before Income Taxes									64,699
Income tax expense									27,030
Net income (loss)									37,669
Noncontrolling interests									3
Allocation of earnings to preferred stockholders									(64)
Net Income Attributable to Common Stockholders									37,608
Consolidated Investment Products [Member]
Condensed Consolidating Financial Information [Line Items]
Total operating revenues
Total operating expenses									148
Operating income (loss)									(148)
Total other non-operating income (expense)									2,649
Income Before Income Taxes									2,501
Income tax expense
Net income (loss)									2,501
Noncontrolling interests
Allocation of earnings to preferred stockholders
Net Income Attributable to Common Stockholders									2,501
Eliminations [Member]
Condensed Consolidating Financial Information [Line Items]
Total operating revenues									167	[1]
Total operating expenses									167	[1]
Operating income (loss)									0	[1]
Total other non-operating income (expense)									(2,397)	[1]
Income Before Income Taxes									(2,397)	[1]
Income tax expense										[1]
Net income (loss)									(2,397)	[1]
Noncontrolling interests									(104)	[1]
Allocation of earnings to preferred stockholders										[1]
Net Income Attributable to Common Stockholders									$ (2,501)	[1]

[1] Adjustments include the elimination of intercompany transactions between the Company and its consolidated sponsored investment products, primarily the elimination of the investments and equity and recording of any noncontrolling interest.