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Fair Value Measurements (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
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 (699) $ (538)
Long-term Debt, Fair Value 1,015,400 1,071,200
Carrying value of long-term debt, excluding derivative liabilities 414,342 379,033
Derivative, Notional Amount 100,000 100,000
Fair Value, Measurements, Recurring [Member]    
Assets:    
Assets held in rabbi trusts 43,756 45,252
Available for sale securities 4,702 4,621
Fair Value Assets 48,458 49,873
Liabilities:    
Fair Value Liabilities (699) (538)
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Assets held in rabbi trusts 27,199 28,589
Available for sale securities 4,702 4,621
Fair Value Assets 31,901 33,210
Liabilities:    
Fair Value Liabilities 0 0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Assets held in rabbi trusts 16,557 16,663
Available for sale securities 0 0
Fair Value Assets 16,557 16,663
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 (699) (538)
Convertible Senior Debentures, Due 2040 [Member]    
Liabilities:    
Embedded derivative (300) (206)
Convertible Senior Debentures, Due 2040 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative (300) (206)
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 (300) (206)
Convertible Senior Debentures, Due 2041 [Member]    
Liabilities:    
Embedded derivative (270) (214)
Convertible Senior Debentures, Due 2041 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative (270) (214)
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 (270) (214)
Convertible Senior Debentures, Due 2042 [Member]    
Liabilities:    
Embedded derivative (129) (118)
Convertible Senior Debentures, Due 2042 [Member] | Fair Value, Measurements, Recurring [Member]    
Liabilities:    
Embedded derivative (129) (118)
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 $ (129) $ (118)