Pension Plans and Other Postretirement Benefit Plans - Change in Benefit Obligation and Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	$ (11,987)	$ (8,826)
Non-current liabilities	(211,880)	(212,006)
Accumulated other comprehensive loss for unfunded benefit obligation for pensions and other postretirement benefit plans	(14,378)	(10,259)
Pension Plan And SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation as of beginning of year	742,382	671,629
Service cost	22,392	19,755	$ 21,614
Interest cost	27,853	28,417	26,096
Actuarial (gain)/loss	74,688	57,829
Benefits paid	(40,400)	(35,248)
Benefit obligation as of end of year	826,915	742,382	671,629
Fair value of plan assets as of beginning of year	642,063	544,051
Actual return on plan assets	96,591	109,942
Employer contributions	22,000	22,000
Benefits paid	(38,630)	(33,930)
Fair value of plan assets as of end of year	722,024	642,063	544,051
Funded status	(104,891)	(100,319)
Other current liabilities	(1,943)	(1,602)
Non-current liabilities	(102,948)	(98,717)
Net amount recognized	(104,891)	(100,319)
Accumulated pension benefit obligation	710,023	644,004
Unrecognized prior service cost	1,902	2,105
Unrecognized net actuarial loss	119,318	114,368
Total	121,220	116,473
Less regulatory asset	(108,301)	(107,395)
Accumulated other comprehensive loss for unfunded benefit obligation for pensions and other postretirement benefit plans	$ 12,919	$ 9,078
Discount rate for benefit obligation	3.25%	3.85%
Discount rate for annual expense	3.85%	4.31%
Expected long-term return on plan assets	5.50%	5.90%
Rate of compensation increase	4.74%	4.66%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation as of beginning of year	$ 159,296	$ 134,053
Service cost	3,902	3,006	3,188
Interest cost	6,042	5,598	4,831
Actuarial (gain)/loss	(2,589)	23,344
Benefits paid	(5,418)	(6,705)
Benefit obligation as of end of year	161,233	159,296	134,053
Fair value of plan assets as of beginning of year	44,853	36,852
Actual return on plan assets	7,320	8,001
Fair value of plan assets as of end of year	52,173	44,853	$ 36,852
Funded status	(109,060)	(114,443)
Other current liabilities	(669)	(640)
Non-current liabilities	(108,391)	(113,803)
Net amount recognized	(109,060)	(114,443)
Unrecognized prior service cost	(3,570)	(4,400)
Unrecognized net actuarial loss	53,737	63,101
Total	50,167	58,701
Less regulatory asset	(48,708)	(57,520)
Accumulated other comprehensive loss for unfunded benefit obligation for pensions and other postretirement benefit plans	$ 1,459	$ 1,181
Discount rate for benefit obligation	3.27%	3.89%
Discount rate for annual expense	3.89%	4.32%
Expected long-term return on plan assets	5.30%	5.70%
Other Postretirement Benefits [Member] | Pre-Age 65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Medical cost trend – initial	6.25%	5.75%
Medical cost trend – ultimate	5.00%	5.00%
Ultimate medical cost trend year	2026	2023
Other Postretirement Benefits [Member] | Post-Age 65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Medical cost trend – initial	6.25%	6.50%
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	$ 72,816
Benefit obligation as of end of year	75,876	$ 72,816
Other Postretirement Benefits [Member] | Fully eligible employees [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation as of beginning of year	34,545
Benefit obligation as of end of year	32,097	34,545
Other Postretirement Benefits [Member] | Other participants [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation as of beginning of year	51,935
Benefit obligation as of end of year	$ 53,260	$ 51,935