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Employee Benefit Plans (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Contributions based on wages paid to covered employees $ 1.6 $ 1.5 $ 1.4
Voluntary contributions to the 401(k) profit-sharing plans and trusts $ 4.4 $ 3.9 $ 3.3