Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012 years	Jun. 30, 2011	Jun. 30, 2012 years	Jun. 30, 2011	Dec. 31, 2011	Dec. 31, 2010
Payments to Acquire Software			$ 18,214	$ 20,537
Cash and cash equivalents	347,494	442,311	347,494	442,311	420,986	496,292
Capitalized software, weighted average remaining life, years	10.0		10.0
Amortization expense related to capitalized software			14,531	12,532
Capitalized software, accumulated amortization	131,631		131,631		117,100
Anti-dilutive employee stock options	16,637,000	7,675,000	14,017,000	7,606,000
Average exercise price of anti-dilutive employee stock options	$ 22.75	$ 28.25	$ 23.40	$ 28.26
SEI Investments (Europe) Limited [Member]
Restricted cash	5,000		5,000		5,000
SEI Investments Distribution Co. (SIDCO) [Member]
Restricted cash	1,000		1,000		1,000
Global Wealth Platform [Member]
Capitalized software, weighted average remaining life, years	10.0		10.0
Amortization expense related to capitalized software			14,297	12,249
Net book value of GWP	302,829		302,829
Capitalized software, accumulated amortization	102,059		102,059
Capitalized software development costs in-progress associated with future releases	9,328		9,328
Software useful life, years			15
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member]
Cash and cash equivalents	$ 185,240		$ 185,240		$ 281,760