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Goodwill (Narrative) (Details) - USD ($)
12 Months Ended
Mar. 31, 2017
Dec. 31, 2017
Jan. 26, 2017
Goodwill [Line Items]      
Goodwill $ 615,252,000 $ 620,333,000  
Goodwill impairment $ 0    
C.B. Fleet Company, Inc.      
Goodwill [Line Items]      
Goodwill     $ 273,058,000