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Fair Value Measurements (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Assets:    
Held-to-maturity Securities, Transferred Security, at Carrying Value $ 0  
Held-to-maturity Securities, Unrecognized Holding Gain 0  
Other than Temporary Impairment Losses, Investments, Held-to-maturity Securities 0  
Liabilities:    
Embedded derivative (731) $ (854)
Long-term Debt, Fair Value 963,100 860,600
Carrying value of long-term debt, excluding derivative liabilities 365,752 367,049
Derivative, Notional Amount 300,000 100,000
Fair Value, Measurements, Recurring [Member]    
Assets:    
Assets held in rabbi trusts 44,262 41,917
Available for sale securities 4,511 3,969
Fair Value Assets 48,773 45,886
Liabilities:    
Fair Value Liabilities (731) (854)
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Assets held in rabbi trusts 28,240 27,297
Available for sale securities 4,511 3,969
Fair Value Assets 32,751 31,266
Liabilities:    
Fair Value Liabilities 0 0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Assets held in rabbi trusts 16,022 14,620
Available for sale securities 0 0
Fair Value Assets 16,022 14,620
Liabilities:    
Fair Value Liabilities 0 0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Assets held in rabbi trusts 0 0
Available for sale securities 0 0
Fair Value Assets 0 0
Liabilities:    
Fair Value Liabilities (731) (854)
Convertible Senior Debentures, Due 2040 [Member]    
Liabilities:    
Embedded derivative (296) (393)
Convertible Senior Debentures, Due 2040 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative (296) (393)
Convertible Senior Debentures, Due 2040 [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative 0 0
Convertible Senior Debentures, Due 2040 [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative 0 0
Convertible Senior Debentures, Due 2040 [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative (296) (393)
Convertible Senior Debentures, Due 2041 [Member]    
Liabilities:    
Embedded derivative (272) (285)
Convertible Senior Debentures, Due 2041 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative (272) (285)
Convertible Senior Debentures, Due 2041 [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative 0 0
Convertible Senior Debentures, Due 2041 [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative 0 0
Convertible Senior Debentures, Due 2041 [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative (272) (285)
Convertible Senior Debentures, Due 2042 [Member]    
Liabilities:    
Embedded derivative (163) (176)
Convertible Senior Debentures, Due 2042 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative (163) (176)
Convertible Senior Debentures, Due 2042 [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative 0 0
Convertible Senior Debentures, Due 2042 [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative 0 0
Convertible Senior Debentures, Due 2042 [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative $ (163) $ (176)