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Debt (Tables)
 3 Months Ended
Mar. 31, 2018
Debt Disclosure [Abstract]  
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments
The following table provides an estimate of the fair value of our long-term debt, including current maturities of long-term debt, using market prices in effect on the valuation date:
 
 
March 31,
 
December 31,
In thousands
 
2018
 
2017
 
2017
Gross long-term debt
 
$
764,700

 
$
726,700

 
$
786,700

Unamortized debt issuance costs
 
(6,418
)
 
(6,990
)
 
(6,813
)
Carrying amount
 
$
758,282

 
$
719,710

 
$
779,887

Estimated fair value(1)
 
$
807,288

 
$
785,980

 
$
853,339


(1) Estimated fair value does not include unamortized debt issuance costs.