CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Parent [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 19,108	$ 16,070
Operating activities:
Net income (loss)	128,571	175,662
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	486,377	355,538
Conservation amortization	82,489	85,847
Deferred income taxes and tax credits, net	5,554	81,899
Net unrealized (gain) loss on derivative instruments	(21,953)	22,957
AFUDC – equity	(12,958)	(11,266)
Monetized production tax credits	(56,177)	0
Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities	11,105	12,358
Funding of pension liability	(13,500)	(18,000)
Regulatory Assets	(22,545)	(85,385)
Other long-term assets and liabilities	(4,872)	(2,068)
Change in certain current assets and liabilities:
Accounts receivable and unbilled revenue	214,166	204,477
Materials and supplies	(7,002)	(2,436)
Increase (Decrease) in Fuel Inventories	7,385	2,789
Prepayments and other	(15,763)	7,391
Purchased gas adjustment	19,911	8,569
Accounts payable	(38,001)	(31,027)
Taxes payable	6,695	(30,074)
Other	(12,379)	(2,983)
Net cash provided by (used in) operating activities	742,333	768,670
Investing activities:
Construction expenditures - excluding equity AFUDC	(760,728)	(761,968)
Other	2,090	5,796
Net cash provided by (used in) investing activities	(758,638)	(756,172)
Financing activities:
Change in short-term debt, net	(123,463)	(106,763)
Dividends paid	(76,728)	(17,543)
Proceeds from long-term debt and bonds issued	642,615	70,583
Redemption of bonds and notes	(450,000)	0
Other	6,228	15,999
Net cash provided by (used in) financing activities	(1,348)	(37,724)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	(17,653)	(25,226)
Supplemental cash flow information:
Cash payments for interest (net of capitalized interest)	234,438	239,566
Cash payments (refunds) for income taxes	7,595	1,649
Non-cash financing and investing activities:
Accounts payable for capital expenditures eliminated from cash flows	105,070	87,456
Subsidiaries [Member]
Parent [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	16,783	15,241
Operating activities:
Net income (loss)	193,709	222,846
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	486,300	355,538
Conservation amortization	82,489	85,847
Deferred income taxes and tax credits, net	12,801	109,015
Net unrealized (gain) loss on derivative instruments	(21,953)	23,098
AFUDC – equity	(12,958)	(11,266)
Monetized production tax credits	(56,177)	0
Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities	3,319	3,946
Funding of pension liability	(13,500)	(18,000)
Regulatory Assets	(22,545)	(85,385)
Other long-term assets and liabilities	5,144	(17,665)
Change in certain current assets and liabilities:
Accounts receivable and unbilled revenue	208,359	215,674
Materials and supplies	(7,002)	(2,436)
Increase (Decrease) in Fuel Inventories	7,385	2,789
Prepayments and other	(15,763)	7,391
Purchased gas adjustment	19,911	8,569
Accounts payable	(37,988)	(31,027)
Taxes payable	7,580	(30,074)
Other	(9,528)	(857)
Net cash provided by (used in) operating activities	814,813	832,425
Investing activities:
Construction expenditures - excluding equity AFUDC	(712,329)	(677,004)
Other	2,090	6,233
Net cash provided by (used in) investing activities	(710,239)	(670,771)
Financing activities:
Change in short-term debt, net	(123,463)	(106,763)
Dividends paid	(151,315)	(96,546)
Proceeds from long-term debt and bonds issued	594,750	0
Redemption of bonds and notes	(450,000)	0
Other	6,228	15,997
Net cash provided by (used in) financing activities	(123,800)	(187,312)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	(19,226)	(25,658)
Supplemental cash flow information:
Cash payments for interest (net of capitalized interest)	152,273	160,426
Cash payments (refunds) for income taxes	13,839	3,058
Non-cash financing and investing activities:
Accounts payable for capital expenditures eliminated from cash flows	$ 105,070	$ 87,456