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Long Term Debt - Other Long Term Debt (Details) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2012
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2012
Dec. 31, 2017
Debt Instrument [Line Items]          
Interest expense, net   $ (8,564) $ (7,820)    
Secured Debt [Member]          
Debt Instrument [Line Items]          
Secured debt $ 50,000 $ 39,500 $ 41,500 $ 50,000 $ 40,000
Debt term 7 years        
Weighted average interest rate   3.10% 2.30%    
Secured Debt [Member] | London Interbank Offered Rate [Member]          
Debt Instrument [Line Items]          
Variable rate       1.50%