CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Operating Activities:
Net income	$ 171,180	$ 155,176	$ 147,334
Non-cash items included in net income:
Depreciation and amortization	265,409	253,142	232,176
Provision for deferred income taxes	(36,837)	(18,231)	11,224
Power and natural gas cost amortizations (deferrals), net	7,122	(78,350)	(51,847)
Amortization of debt expense	2,935	1,974	2,606
Stock-based compensation expense	8,442	8,717	4,713
Equity-related AFUDC	(6,886)	(6,704)	(7,004)
Pension and other postretirement benefit expense	14,283	32,173	29,077
Other regulatory assets and liabilities	(34,189)	(15,129)	4,445
Other non-current assets and liabilities	26,207	(5,280)	(3,769)
Change in decoupling regulatory deferral	(3,328)	33,469	6,056
Realized and unrealized losses (gains) on assets and investments	3,369	(50,006)	(23,187)
Other	(6,066)	11,957	(2,859)
Contributions to defined benefit pension plan	(10,000)	(42,000)	(42,000)
Cash paid on settlement of interest rate swap agreements	(409)	(17,035)	(17,568)
Cash received on settlement of interest rate swap agreements	7,869		324
Changes in certain current assets and liabilities:
Accounts and notes receivable	36,855	(56,007)	(46,107)
Inventory	(52,309)	(22,941)	(17,282)
Collateral posted for derivative instruments	129,226	(141,014)	(17,564)
Income taxes receivable	1,506	(1,125)	20,199
Other current assets	(26,037)	(6,613)	930
Accounts payable	(66,144)	65,928	33,369
Other current liabilities	14,881	22,106	4,074
Net cash provided by operating activities	447,079	124,207	267,340
Investing Activities:
Utility property capital expenditures (excluding equity-related AFUDC)	(498,637)	(451,995)	(439,939)
Issuance of notes receivable	(3,425)	(2,745)	(1,841)
Equity and property investments	(13,380)	(10,642)	(16,001)
Proceeds from sale of investments	3,200	1,000	8,306
Other	1,853	4,144	4,559
Net cash used in investing activities	(510,389)	(460,238)	(444,916)
Financing Activities:
Net increase (decrease) in short-term borrowings	(114,000)	179,000	81,000
Proceeds from issuance of long-term debt	250,000	399,856	140,000
Maturity of long-term debt and finance leases	(16,735)	(253,085)	(2,935)
Issuance of common stock, net of issuance costs	112,308	137,778	89,998
Cash dividends paid	(140,923)	(129,061)	(118,211)
Other	(5,765)	(7,197)	(4,304)
Net cash provided by financing activities	84,885	327,291	185,548
Net increase (decrease) in cash and cash equivalents	21,575	(8,740)	7,972
Cash and cash equivalents at beginning of year	13,428	22,168	14,196
Cash and cash equivalents at end of year	35,003	13,428	22,168
Cash paid (received) during the year:
Interest	131,522	107,468	98,592
Income taxes paid	2,501	2,251	3,652
Income tax refunds	(800)	(86)	(22,330)
Non-cash financing and investing activities:
Accounts payable for capital expenditures	$ 33,691	$ 27,708	$ 23,938