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Summary of Significant Accounting Policies (Calculation Of Basic And Diluted Earnings Per Share) (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Earnings Per Share [Abstract]                      
Basic earnings per common share, Net income attributable to SEI (Numerator) $ 81,379 $ 79,425 $ 86,240 $ 84,611 $ 77,097 $ 83,983 $ 82,813 $ 74,820 $ 331,655 $ 318,713 $ 288,124
Shares used to compute basic earnings per share                 165,725 168,246 171,561
Basic earnings per common share $ 0.49 $ 0.48 $ 0.52 $ 0.51 $ 0.46 $ 0.50 $ 0.49 $ 0.44 $ 2.00 $ 1.89 $ 1.68
Dilutive effect of stock options, Net income attributable to SEI (Numerator)                 $ 0 $ 0 $ 0
Dilutive effect of stock options, shares                 3,873 4,319 4,157
Diluted earnings per common share, Net income attributable to SEI (Numerator)                 $ 331,655 $ 318,713 $ 288,124
Shares used to compute diluted earnings per share                 169,598 172,565 175,718
Diluted earnings per common share $ 0.48 $ 0.47 $ 0.51 $ 0.50 $ 0.45 $ 0.49 $ 0.48 $ 0.43 $ 1.96 $ 1.85 $ 1.64