CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities:
Net income (loss)	$ 146,897	$ 127,550
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	184,512	115,253
Conservation amortization	36,864	34,761
Deferred income taxes and tax credits, net	14,517	55,957
Net unrealized (gain) loss on derivative instruments	(996)	19,147
AFUDC – equity	(3,351)	(2,928)
Monetized production tax credits	(43,586)	0
Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities	3,720	4,214
Funding of pension liability	(4,500)	(9,000)
Regulatory Assets	20,871	(2,073)
Other long-term assets and liabilities	(13,160)	13,543
Change in certain current assets and liabilities:
Accounts receivable and unbilled revenue	49,476	52,860
Materials and supplies	(1,825)	3,304
Fuel and gas inventory	15,543	19,346
Prepayments and other	(1,503)	(4,354)
Purchased gas adjustment	17,612	13,375
Accounts payable	(27,973)	(41,451)
Taxes payable	19,614	29,374
Other	(13,411)	(677)
Net cash provided by (used in) operating activities	399,321	428,201
Investing activities:
Construction expenditures - excluding equity AFUDC	(241,181)	(223,635)
Other	1,570	(3,804)
Net cash provided by (used in) investing activities	(239,611)	(227,439)
Financing activities:
Change in short-term debt, net	41,226	(230,763)
Dividends paid	(30,096)	(8)
Proceeds from long-term debt and bonds issued	13,179	10,663
Redemption of bonds and notes	(193,447)	0
Other	(1,530)	4,605
Net cash provided by (used in) financing activities	(170,668)	(215,503)
Net increase (decrease) in cash and cash equivalents	(10,958)	(14,741)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Beginning of Period	36,761	41,296
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, End of Period	25,803	26,555
Supplemental cash flow information:
Cash payments for interest (net of capitalized interest)	81,736	76,644
Cash payments (refunds) for income taxes	0	0
Accounts payable for capital expenditures eliminated from cash flows	90,169	45,945
Subsidiaries [Member]
Operating activities:
Net income (loss)	163,037	143,092
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	184,490	115,253
Conservation amortization	36,864	34,761
Deferred income taxes and tax credits, net	19,505	65,212
Net unrealized (gain) loss on derivative instruments	(996)	19,288
AFUDC – equity	(3,351)	(2,928)
Monetized production tax credits	(43,586)	0
Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities	1,131	1,410
Funding of pension liability	(4,500)	(9,000)
Regulatory Assets	20,871	(2,073)
Other long-term assets and liabilities	(9,844)	(8,088)
Change in certain current assets and liabilities:
Accounts receivable and unbilled revenue	41,558	44,630
Materials and supplies	(1,825)	3,304
Fuel and gas inventory	15,543	19,346
Prepayments and other	(1,503)	(4,354)
Purchased gas adjustment	17,612	13,375
Accounts payable	(27,973)	(41,451)
Taxes payable	24,515	29,374
Other	(10,694)	1,515
Net cash provided by (used in) operating activities	420,854	422,666
Investing activities:
Construction expenditures - excluding equity AFUDC	(221,099)	(179,622)
Other	1,570	(3,367)
Net cash provided by (used in) investing activities	(219,529)	(182,989)
Financing activities:
Change in short-term debt, net	41,226	(230,763)
Dividends paid	(58,611)	(28,712)
Redemption of bonds and notes	(193,447)	0
Other	(1,529)	4,635
Net cash provided by (used in) financing activities	(212,361)	(254,840)
Net increase (decrease) in cash and cash equivalents	(11,036)	(15,163)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Beginning of Period	36,009	40,899
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, End of Period	24,973	25,736
Supplemental cash flow information:
Cash payments for interest (net of capitalized interest)	52,847	48,989
Cash payments (refunds) for income taxes	0	0
Accounts payable for capital expenditures eliminated from cash flows	$ 90,169	$ 45,945