CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Operating activities:
Net Income (loss)	$ (23,233)	$ 94,936	$ (32,952)	$ 132,154	$ 71,703	$ 99,202
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation & amortization	136,865		164,715		301,681	345,412
Conservation amortization	20,321		20,029		47,714	53,315
Deferred income taxes and tax credits, net					1,741	2,248
Net unrealized (gain) loss on derivative instruments					36,379	15,145
Gain (Loss) on Extinguishment of Debt					13,546	0
AFUDC - equity					(11,668)	(6,591)
Production tax credit utilization					(14,470)	(41,111)
Other non-cash					696	7,427
Regulatory assets and liabilities					(71,268)	(19,061)
increase (decrease) in purchased gas					45,833	(144,917)
Other long term assets and liabilities					(11,114)	(13,238)
Change in certain current assets and liabilities:
Accounts receivable and unbilled revenue					189,072	203,174
Materials and supplies					(15,837)	(3,769)
Fuel and natural gas inventory					793	(5,338)
Prepayments and other					(171)	(85)
Purchased gas adjustment					0	9,921
Accounts payable					(54,136)	(179,533)
Taxes payable					(6,006)	(20,798)
Other					(28,226)	(10,692)
Net cash provided by (used in) operating activities					496,262	290,711
Investing activities:
Construction expenditures - excluding equity AFUDC					(438,477)	(470,335)
Other					104	(3,977)
Net cash provided by (used in) investing activities					(438,373)	(474,312)
Financing activities:
Change in short-term debt, net					(36,000)	160,703
Dividends paid					(45,064)	(36,077)
Proceeds from long-term debt and bonds issued					644,690	20,000
Repayments of Debt					(174,100)	0
Redemption of bonds and notes					(450,000)	0
Other					(7,350)	7,411
Net cash provided by (used in) financing activities					(67,824)	152,037
Net increase (decrease) in cash, cash equivalents, and restricted cash					(9,935)	(31,564)
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	56,211		23,998		56,211	23,998	66,146
Supplemental cash flow information:
Cash payments for interest (net of capitalized interest)					169,832	165,863
Cash payments (refunds) for income taxes					0	5,376
Non-cash financing and investing activities:
Accounts payable for capital expenditures eliminated from cash flows					57,498	73,757
Increase (Decrease) in Regulatory Assets and Liabilities					0	(47,516)
Subsidiaries [Member]
Operating activities:
Net Income (loss)	15,037	111,321	(8,325)	147,302	126,358	138,977
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation & amortization	136,816		164,692		301,587	345,370
Conservation amortization	20,321		20,029		47,714	53,315
Deferred income taxes and tax credits, net					10,267	8,967
Net unrealized (gain) loss on derivative instruments					36,379	15,145
AFUDC - equity					(11,668)	(6,591)
Production tax credit utilization					(14,470)	(41,111)
Other non-cash					(4,582)	2,210
Regulatory assets and liabilities					(71,204)	(19,061)
increase (decrease) in purchased gas					45,833	(144,917)
Other long term assets and liabilities					(5,073)	(9,822)
Change in certain current assets and liabilities:
Accounts receivable and unbilled revenue					186,425	206,228
Materials and supplies					(15,837)	(3,769)
Fuel and natural gas inventory					729	(5,338)
Prepayments and other					(171)	(85)
Purchased gas adjustment					0	9,921
Accounts payable					(52,580)	(179,591)
Taxes payable					(2,060)	(20,679)
Other					(28,724)	(7,893)
Net cash provided by (used in) operating activities					548,923	341,276
Investing activities:
Construction expenditures - excluding equity AFUDC					(428,775)	(451,215)
Other					104	(3,977)
Net cash provided by (used in) investing activities					(428,671)	(455,192)
Financing activities:
Change in short-term debt, net					(36,000)	160,703
Dividends paid					(99,809)	(83,988)
Other					6,373	7,410
Net cash provided by (used in) financing activities					(129,436)	84,125
Net increase (decrease) in cash, cash equivalents, and restricted cash					(9,184)	(29,791)
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	$ 55,707		$ 23,702		55,707	23,702	$ 64,891
Supplemental cash flow information:
Cash payments for interest (net of capitalized interest)					114,027	112,058
Cash payments (refunds) for income taxes					0	9,784
Non-cash financing and investing activities:
Accounts payable for capital expenditures eliminated from cash flows					57,498	73,757
Increase (Decrease) in Regulatory Assets and Liabilities					$ 0	$ (47,516)