Debt (Tables)

12 Months Ended

Dec. 31, 2016

Long-term Debt, Current and Noncurrent [Abstract]

Schedule of Maturities of Long-term Debt [Table Text Block]

In thousands
Year
2017	$	40,000
2018	97,000
2019	30,000
2020	75,000
2021	60,000
Thereafter	424,700

Schedule of Debt [Table Text Block]

In thousands	2016		2015
First Mortgage Bonds
5.15 % Series B due 2016	$	—	$	25,000
7.00 % Series B due 2017	40,000		40,000
1.545 % Series B due 2018	75,000		—
6.60 % Series B due 2018	22,000		22,000
8.31 % Series B due 2019	10,000		10,000
7.63 % Series B due 2019	20,000		20,000
5.37 % Series B due 2020	75,000		75,000
9.05 % Series A due 2021	10,000		10,000
3.176 % Series B due 2021	50,000		50,000
3.542% Series B due 2023	50,000		50,000
5.62 % Series B due 2023	40,000		40,000
7.72 % Series B due 2025	20,000		20,000
6.52 % Series B due 2025	10,000		10,000
7.05 % Series B due 2026	20,000		20,000
3.211 % Series B due 2026	35,000		—
7.00 % Series B due 2027	20,000		20,000
6.65 % Series B due 2027	19,700		19,700
6.65 % Series B due 2028	10,000		10,000
7.74 % Series B due 2030	20,000		20,000
7.85 % Series B due 2030	10,000		10,000
5.82 % Series B due 2032	30,000		30,000
5.66 % Series B due 2033	40,000		40,000
5.25 % Series B due 2035	10,000		10,000
4.00 % due 2042	50,000		50,000
4.136 % Series B due 2046	40,000		—
	726,700		601,700
Less: Current maturities	40,000		25,000
Total long-term debt	$	686,700	$	576,700

Fair Value Of Long Term Debt Table [Text Block]

	December 31,
In thousands	2016			2015
Gross long-term debt	$	726,700		$	601,700
Unamortized debt issuance costs	(7,377		)	(7,282		)
Carrying amount	$	719,323		$	594,418
Estimated fair value	$	793,339		$	667,168