Debt Obligations and Credit Facilities (Details Textual)	6 Months Ended			6 Months Ended						6 Months Ended					6 Months Ended
	Jun. 30, 2012 USD ($)	Jun. 30, 2012 Letter of Credit [Member] USD ($)	Dec. 31, 2011 Letter of Credit [Member] USD ($)	Jun. 30, 2012 Revolving credit facility [Member] USD ($)	Jun. 30, 2012 Multi-currency term loan [Member] USD ($)	Jun. 30, 2012 Multi-currency term loan [Member] AUD	Jun. 30, 2012 Multi-currency term loan [Member] EUR (€)	Jan. 07, 2011 Senior Unsecured Credit Facilities [Member]	Jan. 07, 2011 Senior Unsecured Credit Facilities [Member] Term loan [Member] USD ($)	Jun. 30, 2012 Senior Unsecured Credit Facilities [Member] Revolving credit facility [Member]	Jan. 07, 2011 Senior Unsecured Credit Facilities [Member] Revolving credit facility [Member] USD ($)	Jun. 30, 2012 Senior secured revolving credit facility [Member] USD ($)	Dec. 31, 2011 Senior secured revolving credit facility [Member] USD ($)	Oct. 07, 2011 Senior secured revolving credit facility [Member] USD ($)	Jun. 30, 2012 5.03% [Member] USD ($)	Jun. 30, 2012 6.09% [Member] USD ($)	Jun. 30, 2012 5.82% [Member] USD ($)	Jun. 30, 2012 6.75% [Member] USD ($)	Jun. 30, 2012 3.19% [Member] USD ($)	Jan. 07, 2011 3.19% [Member]
Debt Disclosure (Textual) [Abstract]
Debt Instrument, Interest Rate, Stated Percentage															5.03%	6.09%	5.82%	6.75%		2.50%
Debt Instrument, Face Amount															$ 75,000,000	$ 50,000,000	$ 50,000,000	$ 250,000,000	$ 300,000,000
Debt Instrument, Offering Date															Jun. 30, 2004	Jun. 30, 2006	Nov. 30, 2006	Nov. 30, 2009	Jan. 31, 2011
Debt Instrument, Maturity Date																Jun. 06, 2016	Nov. 08, 2016	Dec. 02, 2019	Jan. 06, 2016
Debt Instrument, Payment Terms															Payable in seven equal annual installments starting June 14, 2008				Term loan issued in January 2011, payable2.5% per quarter through December 31, 2015, final $150,000 payment on January 6,2016
Debt Instrument, Frequency of Periodic Payment															Payable in seven equal annual installments
Credit agreement				150,000,000	275,000,000				300,000,000		250,000,000
Fixed interest rate as a result of interest rate swap																			3.19%
Final principal payment																			150,000,000
Revolving Credit Facility, which remains undrawn, expires in January 2014, and has a commitment fee payable at an annual rate of 0.20% on unused funds	Revolving Credit Facility, which remains undrawn, expires in January 2014, and has a commitment fee payable at an annual rate of 0.20% on unused funds
Credit Facilities contain typical financial covenants, including ones regarding a maximum leverage ratio of 2.5-to-1, minimum fixed charge coverage ratio of 2.5-to-1 and minimum required levels of assets under management and net worth (as defined) of $50 billion and $400 million, respectively										Credit Facilities contain typical financial covenants, including ones regarding a maximum leverage ratio of 2.5-to-1, minimum fixed charge coverage ratio of 2.5-to-1 and minimum required levels of assets under management and net worth (as defined) of $50 billion and $400 million, respectively
Commitment fee payable on unused funds										0.20%
Initial Borrowing Capacity	275,000,000				221,451,000	17,660,000	26,492,000							75,000,000
Additional Borrowing Capacity														150,000,000
Maturity				Dec. 15, 2013	Dec. 23, 2013					Jan. 31, 2014
Eurodollar margin				1.75%	3.00%			1.50%
Outstanding amounts		61,884,000	75,884,000									0	0
Term loan, maturity					4 years
Term loan, amortization period					4 years
Term loan, structuring fee to administrative agent					4,125,000
Maximum leverage ratio									2.5
Minimum fixed charge coverage ratio									2.5
Minimum required levels of assets under management									50,000,000,000
Minimum required levels of assets under net worth									$ 400,000,000