Concentration of Credit Risk and Significant Relationships (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Concentration Risk [Line Items]
Total revenues	$ 206,016	$ 212,406	$ 208,487	$ 201,792	$ 197,593	$ 178,092	$ 161,933	$ 148,223	$ 828,701	$ 685,841	$ 505,578
Total Revenue [Member] | Geographic Concentration Risk [Member]
Concentration Risk [Line Items]
Percentage of total revenue									9.00%	9.00%	7.00%
US Mid-Cap Growth [Member]
Concentration Risk [Line Items]
Total revenues									90,683	76,327	59,841
US Mid-Cap Growth [Member] | Total Revenue [Member] | Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Percentage of total revenue									10.90%	11.10%	11.80%
US Mid-Cap Value [Member]
Concentration Risk [Line Items]
Total revenues									106,463	93,774	73,720
US Mid-Cap Value [Member] | Total Revenue [Member] | Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Percentage of total revenue									12.90%	13.70%	14.60%
Non-US Growth [Member]
Concentration Risk [Line Items]
Total revenues									156,537	116,173	86,367
Non-US Growth [Member] | Total Revenue [Member] | Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Percentage of total revenue									18.90%	16.90%	17.10%
Non-US Value [Member]
Concentration Risk [Line Items]
Total revenues									$ 108,837	$ 88,342	$ 54,851
Non-US Value [Member] | Total Revenue [Member] | Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Percentage of total revenue									13.10%	12.90%	10.90%