CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Operating activities:
Net Income (loss)	$ 9,996	$ 94,936	$ (39,443)	$ 132,154	$ 81,699	$ 59,759
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation & amortization	161,209		138,281		462,890	483,693
Conservation amortization	21,295		17,734		69,009	71,049
Deferred income taxes and tax credits, net					(1,922)	(3,559)
Net unrealized (gain) loss on derivative instruments					(3,563)	29,861
Gain (Loss) on Extinguishment of Debt					13,546	0
AFUDC - equity					(17,772)	(10,071)
Production tax credit utilization					(17,558)	(35,470)
Other non-cash					5,466	1,712
increase (decrease) in purchased gas					30,859	(155,711)
Other long term assets and liabilities					(17,094)	(3,075)
Regulatory assets and liabilities					(90,513)	(46,993)
Funding of pension liability					(18,000)	(18,000)
Change in certain current assets and liabilities:
Accounts receivable and unbilled revenue					188,053	192,365
Materials and supplies					(4,365)	(3,963)
Fuel and natural gas inventory					(4,290)	(8,442)
Prepayments and other					(18,912)	(2,283)
Purchased gas adjustment					0	9,921
Accounts payable					(26,068)	(160,792)
Taxes payable					9,461	(43,141)
Other					(25,086)	(2,727)
Net cash provided by (used in) operating activities					615,840	354,133
Investing activities:
Construction expenditures - excluding equity AFUDC					(678,085)	(709,139)
Other					(925)	(5,914)
Net cash provided by (used in) investing activities					(679,010)	(715,053)
Financing activities:
Change in short-term debt, net					45,000	(310,297)
Dividends paid					(45,412)	(36,756)
Proceeds from long-term debt and bonds issued					644,690	682,151
Repayments of Debt					(150,200)	0
Redemption of bonds and notes					(450,000)	0
Other					(2,525)	10,386
Net cash provided by (used in) financing activities					41,553	345,484
Net increase (decrease) in cash, cash equivalents, and restricted cash					(21,617)	(15,436)
Cash, cash equivalents, and restricted cash at beginning of period		66,146		55,562	66,146	55,562	$ 55,562
Cash, cash equivalents, and restricted cash at end of period	44,529		40,126		44,529	40,126	66,146
Supplemental cash flow information:
Cash payments for interest (net of capitalized interest)					245,183	236,718
Cash payments (refunds) for income taxes					3,820	8,990
Non-cash financing and investing activities:
Accounts payable for capital expenditures eliminated from cash flows					67,321	65,023
Increase (Decrease) in Regulatory Assets and Liabilities					0	(47,534)
Subsidiaries [Member]
Operating activities:
Net Income (loss)	33,062	111,321	(15,257)	147,302	159,420	123,720
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation & amortization	161,155		138,253		462,742	483,623
Conservation amortization	21,295		17,734		69,009	71,049
Deferred income taxes and tax credits, net					7,259	(1,600)
Net unrealized (gain) loss on derivative instruments					(3,563)	29,861
AFUDC - equity					(17,772)	(10,071)
Production tax credit utilization					(17,558)	(35,470)
Other non-cash					(2,441)	(6,167)
increase (decrease) in purchased gas					30,859	(155,711)
Other long term assets and liabilities					(8,391)	4,972
Regulatory assets and liabilities					(90,513)	(46,993)
Funding of pension liability					18,000	18,000
Change in certain current assets and liabilities:
Accounts receivable and unbilled revenue					185,957	196,877
Materials and supplies					(4,365)	(3,963)
Fuel and natural gas inventory					(4,290)	(8,442)
Prepayments and other					(18,912)	(2,283)
Purchased gas adjustment					0	9,921
Accounts payable					(26,038)	(160,850)
Taxes payable					9,542	(40,178)
Other					(22,628)	794
Net cash provided by (used in) operating activities					690,317	431,089
Investing activities:
Construction expenditures - excluding equity AFUDC					(641,862)	(680,118)
Other					(925)	(5,916)
Net cash provided by (used in) investing activities					(642,787)	(686,034)
Financing activities:
Change in short-term debt, net					45,000	(310,297)
Dividends paid					(125,551)	(113,862)
Proceeds from long-term debt and bonds issued					0	443,151
Proceeds from Contributions from Parent			210,000		0	210,000
Other					11,921	11,048
Net cash provided by (used in) financing activities					(68,630)	240,040
Net increase (decrease) in cash, cash equivalents, and restricted cash					(21,100)	(14,905)
Cash, cash equivalents, and restricted cash at beginning of period		$ 64,891		$ 53,493	64,891	53,493	53,493
Cash, cash equivalents, and restricted cash at end of period	$ 43,791		$ 38,588		43,791	38,588	$ 64,891
Supplemental cash flow information:
Cash payments for interest (net of capitalized interest)					161,083	152,571
Cash payments (refunds) for income taxes					9,259	16,540
Non-cash financing and investing activities:
Accounts payable for capital expenditures eliminated from cash flows					67,321	65,023
Increase (Decrease) in Regulatory Assets and Liabilities					$ 0	$ (47,534)