Marketable Securities (Narrative) (Details) (USD $)	12 Months Ended				3 Months Ended		12 Months Ended		3 Months Ended								12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 SEI Private Trust Company [Member] Minimum [Member]	Dec. 31, 2012 SEI Private Trust Company [Member] Maximum [Member]	Dec. 31, 2012 U.S. Government Agency And Commercial Paper Securities [Member]	Dec. 31, 2011 U.S. Government Agency And Commercial Paper Securities [Member]	Dec. 31, 2012 SIV Securities [Member]	Sep. 30, 2012 SIV Securities [Member]	Jun. 30, 2012 SIV Securities [Member]	Mar. 31, 2012 SIV Securities [Member]	Dec. 31, 2011 SIV Securities [Member]	Sep. 30, 2011 SIV Securities [Member]	Jun. 30, 2011 SIV Securities [Member]	Mar. 31, 2011 SIV Securities [Member]	Dec. 31, 2012 SIV Securities [Member]	Dec. 31, 2011 SIV Securities [Member]	Dec. 31, 2010 SIV Securities [Member]	Dec. 31, 2012 Gryphon SIV [Member]	Dec. 31, 2011 Stanfield Victoria SIV [Member]	Dec. 31, 2012 LSV-Sponsored Mutual Funds [Member]	Dec. 31, 2011 LSV-Sponsored Mutual Funds [Member]
Schedule of Investments [Line Items]
Net unrealized holding gains	$ 2,829,000	$ 2,574,000	$ 1,339,000	$ 960,000
Unrealized holding gains during the period, income tax expense	1,554,000	1,518,000
Available-for-sale Securities, Gross Realized Gain (Loss)	0	0	0
Available-for-sale Securities, Debt Maturities, Date					Dec. 15, 2020	Nov. 15, 2041
Net gains from SIV securities																	13,240,000	3,390,000	44,247,000
Gains from cash payments received from SIV securities																	6,776,000	10,614,000	27,510,000
Gains or losses from increase/decrease in fair value of SIV securities																	1,142,000	(7,224,000)	16,460,000
Trading Securities, Realized Gain (Loss)									5,897,000	3,389,000	1,072,000	2,882,000	(738,000)	(792,000)	(1,944,000)	6,864,000				5,322,000	0	1,004,000	0
Securities owned	20,088,000	20,949,000					20,088,000	20,949,000
Realized Investment Gains (Losses)							$ 0	$ 0