Summary of Significant Accounting Policies - Additional Information (Details) (USD $)	12 Months Ended					0 Months Ended	12 Months Ended
	Dec. 31, 2013 segment	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011 Term Loan	Jul. 03, 2006 Term Loan	Aug. 16, 2012 Interest Rate Swap	Dec. 31, 2013 Class B Liability Awards	Dec. 31, 2013 Excluding APAM
Significant Accounting Policies [Line Items]
Partnership distributions	$ 332,000,000	$ 135,000,000	$ 122,800,000					$ 290,500,000
Accrued fees to authorized agents	5,400,000	3,600,000
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	5 years						5 years
Debt principal amount				400,000,000	400,000,000
Payments to settle derivative	$ 0	$ 1,135,000	$ 0			$ 1,100,000
Portfolio Manager Performance Period	3 years
Number of Operating Segments	1