CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Activities:
Net income	$ 129,488	$ 196,763	$ 136,598
Non-cash items included in net income:
Depreciation and amortization	224,223	205,994	187,318
Provision for deferred income taxes	44,964	15,098	8,570
Power and natural gas cost amortizations (deferrals), net	(9,923)	(45,917)	10,263
Amortization of debt expense	3,237	2,680	2,967
Amortization of investment in exchange power		1,633	2,450
Stock-based compensation expense	5,846	11,353	5,367
Equity-related AFUDC	(6,970)	(6,585)	(6,554)
Pension and other postretirement benefit expense	33,812	36,417	32,017
Other regulatory assets and liabilities and deferred debits and credits	10,287	65	27,512
Change in decoupling regulatory deferral	2,971	(10,327)	(1,288)
Gain on sale of investments	(4,795)	(7,450)
Other	1,998	(13,526)	1,114
Contributions to defined benefit pension plan	(22,000)	(22,000)	(22,000)
Cash paid on settlement of interest rate swap agreements	(33,499)	(13,325)	(32,174)
Cash received on settlement of interest rate swap agreements			5,594
Changes in certain current assets and liabilities:
Accounts and notes receivable	(10,960)	(4,366)	15,474
Materials and supplies, fuel stock and stored natural gas	(868)	(6,148)	(5,807)
Collateral posted for derivative instruments	1,579	63,974	(4,128)
Income taxes receivable	(41,363)	(8,736)	2,021
Other current assets	(2,401)	(3,657)	(2,589)
Accounts payable	(10,152)	7,471	(470)
Other current liabilities	15,530	(1,199)	(370)
Net cash provided by operating activities	331,004	398,212	361,885
Investing Activities:
Utility property capital expenditures (excluding equity-related AFUDC)	(404,306)	(442,510)	(424,350)
Issuance of notes receivable by subsidiaries	(4,393)	(7,303)	(3,555)
Equity and property investments made by subsidiaries	(5,925)	(13,508)	(13,283)
Proceeds from sale of investments	6,786	16,407
Other	(2,905)	1,403	756
Net cash used in investing activities	(410,743)	(445,511)	(440,432)
Financing Activities:
Net increase (decrease) in short-term borrowings	17,200	(4,200)	84,603
Proceeds from issuance of long-term debt	165,000	180,000	374,621
Maturity of long-term debt and finance leases	(54,800)	(92,660)	(277,438)
Issuance of common stock, net of issuance costs	72,200	64,573	1,207
Cash dividends paid	(110,254)	(102,772)	(98,046)
Other	(5,307)	(2,402)	(7,916)
Net cash provided by financing activities	84,039	42,539	77,031
Net increase (decrease) in cash and cash equivalents	4,300	(4,760)	(1,516)
Cash and cash equivalents at beginning of year	9,896	14,656	16,172
Cash and cash equivalents at end of year	14,196	9,896	14,656
Cash paid (received) during the year:
Interest	97,717	99,060	97,437
Income taxes paid	1,901	26,764	17,801
Income tax refunds	(918)	(979)	(3,025)
Non-cash financing and investing activities:
Accounts payable for capital expenditures	$ 32,039	$ 25,644	$ 31,868