Consolidation of Sponsored Investment Products - Consolidated Statement of Operation (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Condensed Consolidating Financial Information [Line Items]
Total operating revenues	$ 96,140	$ 66,818	$ 182,308	$ 129,216
Total operating expenses	69,258	52,565	134,326	107,010
Operating income (loss)	26,882	14,253	47,982	22,206
Total other non-operating (expense) income	(2,781)		(1,246)
Income Before Income Taxes	24,101	13,936	46,736	23,018
Income tax expense	9,318	5,569	17,768	9,201
Net income (loss)	14,783	8,367	28,968	13,817
Noncontrolling interests	602		377
Net Income Attributable to Common Stockholders	15,385	8,367	29,345	13,753
Balance Before Consolidation of Investment Products [Member]
Condensed Consolidating Financial Information [Line Items]
Total operating revenues	96,135		182,256
Total operating expenses	69,125		134,010
Operating income (loss)	27,010		48,246
Total other non-operating (expense) income	(2,321)		(1,165)
Income Before Income Taxes	24,689		47,081
Income tax expense	9,318		17,768
Net income (loss)	15,371		29,313
Noncontrolling interests	14		32
Net Income Attributable to Common Stockholders	15,385		29,345
Consolidated Investment Products [Member]
Condensed Consolidating Financial Information [Line Items]
Total operating revenues
Total operating expenses	128		264
Operating income (loss)	(128)		(264)
Total other non-operating (expense) income	(2,543)		(1,691)
Income Before Income Taxes	(2,671)		(1,955)
Income tax expense
Net income (loss)	(2,671)		(1,955)
Noncontrolling interests
Net Income Attributable to Common Stockholders	(2,671)		(1,955)
Eliminations and Adjustments [Member]
Condensed Consolidating Financial Information [Line Items]
Total operating revenues	5		52
Total operating expenses	5		52
Operating income (loss)
Total other non-operating (expense) income	2,083		1,610
Income Before Income Taxes	2,083		1,610
Income tax expense
Net income (loss)	2,083		1,610
Noncontrolling interests	588		345
Net Income Attributable to Common Stockholders	$ 2,671		$ 1,955