Unaudited Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
CASH FLOWS FROM OPERATIONS:
Net income	$ 42,047			$ 40,299			$ 98,041	$ 95,405
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	64,797			62,359			190,880	184,166
Amortization of deferred financing costs and debt discounts							2,950	3,006
Deferred income taxes and investment tax credit adjustments - net							6,677	(2,001)
Allowance for equity funds used during construction							(4,133)	(3,116)
Loss on early extinguishment of debt	0			0			0	2,415	$ 2,400
Change in certain assets and liabilities:
Accounts receivable							(12,443)	2,530
Inventories							15,686	(16,652)
Accounts payable							25,221	(24,228)
Accrued and other current liabilities							(4,890)	(9,041)
Accrued taxes payable/receivable							405	9,904
Accrued interest							10,869	8,861
Pension and other postretirement benefit assets and liabilities							(12,426)	(5,179)
Short-term and long-term regulatory assets and liabilities							(43,090)	828
Prepayments and other current assets							9,519	6,159
Other - net							(7,387)	421
Net cash provided by operating activities							256,841	241,160
CASH FLOWS FROM INVESTING ACTIVITIES:
Capital expenditures							(197,383)	(149,369)
Project development costs							(977)	(1,848)
Cost of removal and regulatory recoverable ARO payments							(31,946)	(29,483)
Other							0	119
Net cash used in investing activities							(230,306)	(180,581)
CASH FLOWS FROM FINANCING ACTIVITIES:
Borrowings under revolving credit facilities							145,000	80,000
Repayments under revolving credit facilities							(85,000)	(30,000)
Long-term borrowings, net of discount							95,000	474,568
Retirement of long-term debt, including early payment premium							(95,000)	(472,135)
Distributions to shareholders							96,102	93,993
Preferred dividends of subsidiary	(803)	$ (804)	$ (803)	(803)	$ (804)	$ (803)	(2,410)	(2,410)
Deferred financing costs paid							(1,384)	(5,664)
Payments for financed capital expenditures							36	51
Other							(94)	(118)
Net cash used in financing activities							(40,026)	(49,803)
Net change in cash, cash equivalents and restricted cash							(13,491)	10,776
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 13,131			$ 59,328			13,131	59,328	$ 26,622	$ 48,552
Cash paid during the period for:
Interest (net of amount capitalized)							77,084	81,073
Income Taxes Paid, Net							21,400	27,000
Non-cash investing activities:
Capital Expenditures Incurred but Not yet Paid							$ 67,921	$ 49,569