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Fair Value Measurements Narrative (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2016
Dec. 31, 2015
Jun. 27, 2012
Aug. 25, 2010
Level 1          
Fair Value Measurements [Line Items]          
Defined benefit plan, transfers between measurement levels $ 0 $ 0      
Level 2          
Fair Value Measurements [Line Items]          
Defined benefit plan, transfers between measurement levels 0 0      
Level 3          
Fair Value Measurements [Line Items]          
Defined benefit plan, transfers between measurement levels 0 0      
Senior | $400 Million Senior Notes          
Fair Value Measurements [Line Items]          
Debt instrument face amount 400,000,000 400,000,000 $ 400,000,000 $ 400,000,000  
Senior | $400 Million Senior Notes | Level 2          
Fair Value Measurements [Line Items]          
Debt instrument fair value 439,400,000 439,400,000 432,000,000    
Senior | $250 Million Senior Notes          
Fair Value Measurements [Line Items]          
Debt instrument face amount 250,000,000 250,000,000 250,000,000   $ 250,000,000
Senior | $250 Million Senior Notes | Level 2          
Fair Value Measurements [Line Items]          
Debt instrument fair value $ 272,900,000 $ 272,900,000 $ 267,700,000