7. DEBT

Short-Term Debt

At September 30, 2018, NW Natural had short-term debt of $100.5 million, which was comprised entirely of commercial paper. The carrying cost of 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 September 30, 2018, NW Natural's commercial paper had a maximum remaining maturity of 12 days and average remaining maturity of 7 days.

Long-Term Debt

At September 30, 2018, NW Natural had long-term debt of $809.6 million, which included $5.9 million of unamortized debt issuance costs. Utility long-term debt consists of first mortgage bonds (FMBs) with maturity dates ranging from 2018 through 2048, interest rates ranging from 1.545% to 9.05%, and a weighted average coupon rate of 4.690%. In March 2018, NW Natural retired $22.0 million of FMBs with a coupon rate of 6.60%, and in September 2018, NW Natural issued $50.0 million of FMBs with a coupon rate of 4.110%, due in 2048.

Fair Value of Long-Term Debt

NW Natural's outstanding debt does not trade in active markets. NW Natural estimates the fair value of long-term debt using utility companies with similar credit ratings, terms, and remaining maturities to NW Natural's 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 long-term debt, including current maturities of long-term debt, using market prices in effect on the valuation date:



	September 30,						December 31,
In thousands	2018			2017			2017
Gross long-term debt	$	815,534		$	786,700		$	786,700
Unamortized debt issuance costs	(5,940		)	(7,276		)	(6,813		)
Carrying amount	$	809,594		$	779,424		$	779,887
Estimated fair value⁽¹⁾	$	833,962		$	847,068		$	853,339

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