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Defined Contribution Plans (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]      
Employer matching contribution, percentage of employees' pay 4.00%    
Contributions paid $ 15.9 $ 11.4 $ 10.1
Minimum [Member]      
Defined Contribution Plan Disclosure [Line Items]      
Percentage Contributed for Additional Performance Target 1.00%    
Maximum [Member]      
Defined Contribution Plan Disclosure [Line Items]      
Percentage Contributed for Additional Performance Target 2.00%