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Debt - Long-Term Debt (Details) - USD ($)
$ in Thousands
Feb. 16, 2018
Dec. 31, 2017
Dec. 31, 2016
Nov. 21, 2014
Debt Instrument [Line Items]        
4.650% Senior Notes due 2024 $ 300,000 $ 500,000 $ 500,000 $ 500,000
Unamortized Debt Issuance Expense   (4,698) (5,385)  
Fair Value Hedge Liabilities   (2,990) (1,557)  
Term Loan Facility   300,000 300,000  
Revolving credit facility   0 0  
Long-term Debt   $ 792,312 $ 793,058  
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate       4.65%