Debt (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

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

In thousands
Year
2018	$	97,000
2019	30,000
2020	75,000
2021	60,000
2022	—
Thereafter	524,700

Schedule of Debt [Table Text Block]

In thousands	2017		2016
First Mortgage Bonds
7.000 % Series B due 2017	$	—	$	40,000
1.545 % Series B due 2018	75,000		75,000
6.600 % Series B due 2018	22,000		22,000
8.310 % Series B due 2019	10,000		10,000
7.630 % Series B due 2019	20,000		20,000
5.370 % Series B due 2020	75,000		75,000
9.050 % 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
5.620 % Series B due 2023	40,000		40,000
7.720 % Series B due 2025	20,000		20,000
6.520 % Series B due 2025	10,000		10,000
7.050 % Series B due 2026	20,000		20,000
3.211 % Series B due 2026	35,000		35,000
7.000 % Series B due 2027	20,000		20,000
2.822 % Series B due 2027	25,000		—
6.650 % Series B due 2027	19,700		19,700
6.650 % Series B due 2028	10,000		10,000
7.740 % Series B due 2030	20,000		20,000
7.850 % Series B due 2030	10,000		10,000
5.820 % Series B due 2032	30,000		30,000
5.660 % Series B due 2033	40,000		40,000
5.250 % Series B due 2035	10,000		10,000
4.000 % Series B due 2042	50,000		50,000
4.136 % Series B due 2046	40,000		40,000
3.685 % Series B due 2047	75,000		—
	786,700		726,700
Less: Current maturities	97,000		40,000
Total long-term debt	$	689,700	$	686,700

Fair Value Of Long Term Debt Table [Text Block]

	December 31,
In thousands	2017			2016
Gross long-term debt	$	786,700		$	726,700
Unamortized debt issuance costs	(6,813		)	(7,377		)
Carrying amount	$	779,887		$	719,323
Estimated fair value	$	853,339		$	793,339