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Employee Benefit Plans - Annual Retirement Plan Expense Assumptions (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.26% 4.55%  
Minimum      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate     4.01%
Long-term rate of return on plan assets 4.20% 4.50% 4.10%
Maximum      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate     4.53%
Long-term rate of return on plan assets 4.30% 4.65% 6.10%