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Business Segment Information (Schedule of Revenues And Assets Based On Location) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Revenues $ 408,219 $ 386,018 $ 372,331 $ 359,984 $ 368,810 $ 354,641 $ 343,831 $ 334,263 $ 1,526,552 $ 1,401,545 $ 1,334,208
Assets 1,853,369       1,636,823       1,853,369 1,636,823  
United States [Member]                      
Revenues                 1,298,381 1,191,640 1,123,165
Assets 1,471,260       1,317,235       1,471,260 1,317,235  
International Operations [Member]                      
Revenues                 228,171 209,905 $ 211,043
Assets $ 382,109       $ 319,588       $ 382,109 $ 319,588