Pension Plans and Other Postretirement Benefit Plans - Change in Benefit Obligation and Plan Assets (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities		$ (13,582)	$ (11,987)
Non-current liabilities		(153,467)	(211,880)
Accumulated other comprehensive loss for unfunded benefit obligation for pensions and other postretirement benefit plans		(11,039)	(14,378)
Pension Plan And SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation as of beginning of year		826,915	742,382
Service cost	[1]	25,306	22,392	$ 19,755
Interest cost		26,160	27,853	28,417
Actuarial (gain)/loss		(13,997)	74,688
Benefits paid		(65,342)	(40,400)
Benefit obligation as of end of year		799,042	826,915	742,382
Fair value of plan assets as of beginning of year		722,024	642,063
Actual return on plan assets		50,370	96,591
Employer contributions		42,000	22,000
Benefits paid		(63,431)	(38,630)
Fair value of plan assets as of end of year		750,963	722,024	642,063
Funded status		(48,079)	(104,891)
Other current liabilities		(1,951)	(1,943)
Non-current liabilities		(46,128)	(102,948)
Net amount recognized		(48,079)	(104,891)
Accumulated pension benefit obligation		685,493	710,023
Unrecognized prior service cost (credit)		1,699	1,902
Unrecognized net actuarial loss		94,109	119,318
Total		95,808	121,220
Less regulatory asset		(85,550)	(108,301)
Accumulated other comprehensive loss for unfunded benefit obligation for pensions and other postretirement benefit plans		$ 10,258	$ 12,919
Discount rate for benefit obligation		3.39%	3.25%
Discount rate for annual expense		3.25%	3.85%
Expected long-term return on plan assets		5.40%	5.50%
Rate of compensation increase		4.66%	4.74%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation as of beginning of year		$ 161,233	$ 159,296
Service cost	[1]	4,114	3,902	3,006
Interest cost		5,139	6,042	5,598
Actuarial (gain)/loss		(2,808)	(2,589)
Benefits paid		(5,696)	(5,418)
Benefit obligation as of end of year		167,598	161,233	159,296
Fair value of plan assets as of beginning of year		52,173	44,853
Actual return on plan assets		7,371	7,320
Fair value of plan assets as of end of year		59,544	52,173	$ 44,853
Funded status		(108,054)	(109,060)
Other current liabilities		(684)	(669)
Non-current liabilities		(107,370)	(108,391)
Net amount recognized		(108,054)	(109,060)
Unrecognized prior service cost (credit)		(2,741)	(3,570)
Unrecognized net actuarial loss		48,872	53,737
Total		46,131	50,167
Less regulatory asset		(45,350)	(48,708)
Accumulated other comprehensive loss for unfunded benefit obligation for pensions and other postretirement benefit plans		$ 781	$ 1,459
Discount rate for benefit obligation		3.40%	3.27%
Discount rate for annual expense		3.27%	3.89%
Expected long-term return on plan assets		4.60%	5.30%
Other Postretirement Benefits [Member] | Pre-Age 65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Medical cost trend – initial		6.00%	6.25%
Medical cost trend – ultimate		5.00%	5.00%
Ultimate medical cost trend year		2026	2026
Other Postretirement Benefits [Member] | Post-Age 65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Medical cost trend – initial		6.00%	6.25%
Medical cost trend – ultimate		5.00%	5.00%
Ultimate medical cost trend year		2026	2026
Other Postretirement Benefits [Member] | Retirees [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation as of beginning of year		$ 75,876
Benefit obligation as of end of year		78,347	$ 75,876
Other Postretirement Benefits [Member] | Fully eligible employees [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation as of beginning of year		32,097
Benefit obligation as of end of year		32,144	32,097
Other Postretirement Benefits [Member] | Other participants [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation as of beginning of year		53,260
Benefit obligation as of end of year		$ 57,107	$ 53,260

[1] Total service costs in the table above are recorded to the same accounts as labor expense. Labor and benefits expense is recorded to various projects based on whether the work is a capital project or an operating expense. Approximately 40 percent of all labor and benefits is capitalized to utility property and 60 percent is expensed to utility other operating expenses.