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Business Acquisitions- WIL Research Pro Forma Financial Information (Details) - WIL Research - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended
Sep. 24, 2016
Sep. 26, 2015
Sep. 24, 2016
Sep. 26, 2015
Business Acquisition [Line Items]        
Revenue $ 425,720 $ 402,744 $ 1,275,175 $ 1,166,466
Net income attributable to common shareholders $ 39,530 $ 40,931 $ 129,050 $ 116,863
Earnings per common share        
Basic (in dollars per share) $ 0.84 $ 0.88 $ 2.75 $ 2.51
Diluted (in dollars per share) $ 0.82 $ 0.87 $ 2.70 $ 2.46