Short-Term Debt

At March 31, 2018, we had short-term debt of $50.0 million, which was comprised entirely of commercial paper. The carrying cost of our commercial paper approximates fair value using Level 2 inputs. See Note 2 in the 2017 Form 10-K for a description of the fair value hierarchy. At March 31, 2018, our commercial paper had a maximum remaining maturity of 51 days and average remaining maturity of 31 days.

Long-Term Debt

At March 31, 2018, we had long-term debt of $758.3 million, which included $6.4 million of unamortized debt issuance costs. Utility long-term debt consists of first mortgage bonds (FMBs) with maturity dates ranging from 2018 through 2047, interest rates ranging from 1.545% to 9.05%, and a weighted average coupon rate of 4.728%. In March 2018, we retired $22.0 million of FMBs with a coupon rate of 6.60%.

Fair Value of Long-Term Debt

Our outstanding debt does not trade in active markets. We estimate the fair value of our long-term debt using utility companies with similar credit ratings, terms, and remaining maturities to our long-term debt that actively trade in public markets. These valuations are based on Level 2 inputs as defined in the fair value hierarchy. See Note 2 in the 2017 Form 10-K for a description of the fair value hierarchy.

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.