Pension and Other Postretirement Benefits (Tables)

12 Months Ended

Dec. 31, 2017

Pension and Other Postretirement Benefits Cost (Reversal of Cost) [Abstract]

Schedule of Defined Benefit Plans Disclosures [Table Text Block]

	Postretirement Benefit Plans
	Pension Benefits						Other Benefits
In thousands	2017			2016			2017			2016
Reconciliation of change in benefit obligation:
Obligation at January 1	$	457,839		$	445,628		$	29,395		$	31,049
Service cost	7,090			7,083			341			391
Interest cost	18,111			18,399			1,141			1,175
Net actuarial (gain) loss	34,829			7,688			(213		)	(1,488		)
Benefits paid(1)	(31,580		)	(20,959		)	(1,737		)	(1,732		)
Obligation at December 31	$	486,289		$	457,839		$	28,927		$	29,395
Reconciliation of change in plan assets:
Fair value of plan assets at January 1	$	257,714		$	249,338		$	—		$	—
Actual return on plan assets	40,308			12,593			—			—
Employer contributions	21,483			16,742			1,737			1,732
Benefits paid(1)	(31,580		)	(20,959		)	(1,737		)	(1,732		)
Fair value of plan assets at December 31	$	287,925		$	257,714		$	—		$	—
Funded status at December 31	$	(198,364	)	$	(200,125	)	$	(28,927	)	$	(29,395	)

Schedule Of Amounts Recognized In Regulatory Assets Or Other Comprehensive Income Loss [Table Text Block]

	Regulatory Assets																		Other Comprehensive Loss (Income)
	Pension Benefits									Other Postretirement Benefits									Pension Benefits
In thousands	2017			2016			2015			2017			2016			2015			2017			2016			2015
Net actuarial loss (gain)	$	12,177		$	14,005		$	419		$	(214	)	$	(1,488	)	$	2,724		$	2,777		$	(1,196	)	$	(2,549	)
Settlement Loss	—			—			—			—			—			—			—			193			—
Amortization of:
Prior service cost	(127		)	(230		)	(230		)	468			468			(197		)	—			—			—
Actuarial loss	(14,802		)	(13,238		)	(16,372		)	(696		)	(705		)	(554		)	(946		)	1,386			(2,236		)
Total	$	(2,752	)	$	537		$	(16,183	)	$	(442	)	$	(1,725	)	$	1,973		$	1,831		$	383		$	(4,785	)

Schedule Of Defined Benefit Plan Amounts In AOCL And Regulatory Assets [Table Text Block]

	Regulatory Assets										AOCL
	Pension Benefits				Other Postretirement Benefits						Pension Benefits
In thousands	2017		2016		2017			2016			2017		2016
Prior service cost (credit)	$	49	$	176	$	(2,206	)	$	(2,675	)	$	—	$	1
Net actuarial loss	175,035		177,660		6,964			7,874			13,266		11,434
Total	$	175,084	$	177,836	$	4,758		$	5,199		$	13,266	$	11,435

Schedule of Amounts Recognized in Other Comprehensive Income (Loss) [Table Text Block]

	Year Ended December 31,
In thousands	2017			2016
Beginning balance	$	(6,951	)	$	(7,162	)
Amounts reclassified to AOCL	(2,794		)	(1,196		)
Amounts reclassified from AOCL:
Amortization of actuarial losses	946			1,386
Loss from plan settlement	—			193
Total reclassifications before tax	(1,848		)	383
Tax expense (benefit)	361			(172		)
Total reclassifications for the period	(1,487		)	211
Ending balance	$	(8,438	)	$	(6,951	)

Schedule of Allocation of Plan Assets [Table Text Block]

Asset Category	Target Allocation
U.S. large cap equity	29.3	%
U.S. small/mid cap equity	6.9
Non-U.S. equity	28.0
Emerging markets equity	11.8
Long government/credit	17.5
High yield bonds	2.0
Emerging market debt	3.5
Real estate funds	1.0

Schedule of Net Benefit Costs [Table Text Block]

	Pension Benefits									Other Postretirement Benefits
In thousands	2017			2016			2015			2017			2016			2015
Service cost	$	7,090		$	7,083		$	8,267		$	341		$	391		$	527
Interest cost	18,111			18,399			18,360			1,141			1,175			1,179
Expected return on plan assets	(20,433		)	(20,054		)	(20,676		)	—			—			—
Amortization of prior service costs	127			231			231			(468		)	(468		)	197
Amortization of net actuarial loss	15,748			14,624			18,609			696			705			554
Settlement expense	—			193			—			—			—			—
Net periodic benefit cost	20,643			20,476			24,791			1,710			1,803			2,457
Amount allocated to construction	(6,597		)	(5,746		)	(6,834		)	(587		)	(600		)	(808		)
Amount deferred to regulatory balancing account(1)	(6,542		)	(6,252		)	(8,241		)	—			—			—
Net amount charged to expense	$	7,504		$	8,478		$	9,716		$	1,123		$	1,203		$	1,649

Sensitivity Analysis Of Retirement Benefit Costs and Benefit Obligations [Table Text Block]

	Pension Benefits						Other Postretirement Benefits
	2017		2016		2015		2017		2016		2015
Assumptions for net periodic benefit cost:
Weighted-average discount rate	3.99	%	4.17	%	3.82	%	3.85	%	4.00	%	3.74	%
Rate of increase in compensation	3.25-4.5%		3.25-4.5%		3.25-5.0%		n/a		n/a		n/a
Expected long-term rate of return	7.50	%	7.50	%	7.50	%	n/a		n/a		n/a
Assumptions for year-end funded status:
Weighted-average discount rate	3.52	%	4.00	%	4.21	%	3.44	%	3.85	%	4.00	%
Rate of increase in compensation	3.25-4.5%		3.25-4.5%		3.25-4.5%		n/a		n/a		n/a
Expected long-term rate of return	7.50	%	7.50	%	7.50	%	n/a		n/a		n/a

Schedule of Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates [Table Text Block]

In thousands	1% Increase		1% Decrease
Effect on net periodic postretirement health care benefit cost	$	44	$	(39	)
Effect on the accumulated postretirement benefit obligation	478		(428		)

Defined Benefit Plan Estimated Future Employer Contributions Benefit Payments and Estimated Future Payments [Table Text Block]

In thousands	Pension Benefits		Other Benefits
Employer Contributions:
2016	$	16,742	$	1,732
2017	21,483		1,737
2018 (estimated)	17,710		1,835
Benefit Payments:
2015	35,923		2,018
2016	20,959		1,732
2017	31,580		1,737
Estimated Future Benefit Payments:
2018	22,679		1,835
2019	23,546		1,871
2020	24,542		1,861
2021	25,471		1,904
2022	26,095		1,886
2023-2027	145,065		9,261

Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]

In thousands	December 31, 2017
Investments	Level 1		Level 2		Level 3		Non-Published NAV(1)		Total
U.S. large cap equity	$	—	$	—	$	—	$	102,851	$	102,851
U.S. small/mid cap equity	—		—		—		16,423		16,423
Non-U.S. equity	21,211		—		—		56,075		77,286
Emerging markets equity	—		—		—		28,743		28,743
Fixed income	—		—		—		2,781		2,781
Long government/credit	—		—		—		33,081		33,081
High yield bonds	—		—		—		2,777		2,777
Emerging market debt	—		—		—		12,605		12,605
Real estate	—		—		—		5,544		5,544
Absolute return strategy	—		—		—		189		189
Cash and cash equivalents	82		—		—		5,533		5,615
Total investments	$	21,293	$	—	$	—	$	266,602	$	287,895
	December 31, 2016
Investments	Level 1		Level 2		Level 3		Non-Published NAV(1)		Total
U.S. large cap equity	$	49,841	$	—	$	—	$	5,655	$	55,496
U.S. small/mid cap equity	18,629		—		—		10,232		28,861
Non-U.S. equity	22,404		—		—		25,346		47,750
Emerging markets equity	—		—		—		13,457		13,457
Fixed income	—		—		—		6,719		6,719
Long government/credit	—		34,955		—		17,960		52,915
High yield bonds	—		—		—		14,072		14,072
Emerging market debt	—		—		—		8,504		8,504
Real estate	17,857		—		—		882		18,739
Absolute return strategy	—		—		—		3,111		3,111
Cash and cash equivalents	$	9	$	—	$	—	$	2,482	$	2,491
Total investments	$	108,740	$	34,955	$	—	$	108,420	$	252,115
							December 31,
							2017		2016
Receivables:
Accrued interest and dividend income							$	30	$	451
Due from broker for securities sold							—		5,170
Total receivables							$	30	$	5,621
Liabilities:
Due to broker for securities purchased							$	—	$	22
Total investment in retirement trust							$	287,925	$	257,714

⁽¹⁾  The fair value for these investments is determined using Net Asset Value per share (NAV) as of December 31, as a practical expedient, and therefore they are not classified within the fair value hierarchy. These investments primarily consist of institutional investment products, for which the NAV is generally not publicly available.