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Fair Value Measurements Narrative (Details) - USD ($)
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Level 1    
Fair Value Measurements [Line Items]    
Defined benefit plan, transfers between measurement levels $ 0  
Level 2    
Fair Value Measurements [Line Items]    
Defined benefit plan, transfers between measurement levels 0  
Level 3    
Fair Value Measurements [Line Items]    
Defined benefit plan, transfers between measurement levels 0  
Senior | $250 Million Senior Notes    
Fair Value Measurements [Line Items]    
Debt instrument face amount 250,000,000 $ 250,000,000
Senior | $250 Million Senior Notes | Level 2    
Fair Value Measurements [Line Items]    
Debt instrument fair value 261,800,000 269,200,000
Senior | $400 Million Senior Notes    
Fair Value Measurements [Line Items]    
Debt instrument face amount 400,000,000 400,000,000
Senior | $400 Million Senior Notes | Level 2    
Fair Value Measurements [Line Items]    
Debt instrument fair value $ 424,900,000 $ 440,100,000