Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Operating Activities:
Net Income	$ 259,560	$ 245,872	$ 237,968
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	173,555	179,444	175,941
Deferred income taxes and investment tax credits	(511)	23,901	25,175
Changes in regulatory assets and liabilities	(79,693)	(33,705)	(36,246)
Pension and postretirement benefit plan expense	29,286	33,817	28,970
Contributions to pension and postretirement benefit plans	(44,192)	(44,220)	(45,161)
Earnings of unconsolidated equity-method investments	11,511	11,435	11,513
Distributions from unconsolidated equity-method investments	11,586	11,711	14,477
Allowance for equity funds used during construction	(37,285)	(31,537)	(29,551)
Other non-cash adjustments to net income, net	14,892	8,929	10,457
Change in:
Accounts receivable	(81,545)	(9,434)	(7,630)
Accounts payable and other accrued liabilities	(2,156)	(6,581)	(5,377)
Taxes accrued/receivable	(11,626)	991	17,543
Other current assets	112,602	17,700	(356)
Other current liabilities	(4,628)	(17,885)	8,950
Other assets	22,951	(4,304)	4,484
Net cash provided by operating activities	351,285	363,264	388,131
Investing Activities:
Additions to property, plant and equipment	(432,589)	(299,999)	(310,938)
Payments received from transmission project joint funding partners	17,778	5,876	3,197
Payments to Acquire Partners Interest in Real Estate Partnership, Net of Cash Acquired	(9,881)	(15,148)	(14,338)
Proceeds from Equity Method Investment, Distribution, Return of Capital	8,489	14,439	1,073
Payments to Acquire Equity Securities, FV-NI	(45,572)	(17,186)	(33,382)
Debt Securities, Held-to-Maturity, Purchase	(31,224)	0	0
Proceeds from Sale of Debt and Equity Securities, FV-NI, Held-for-investment	63,857	11,328	25,795
Payments to Acquire Short-term Investments	25,000	25,000	25,000
Proceeds from Maturities, Prepayments and Calls of Short-term Investments	25,000	50,000	0
Other	4,875	2,037	6,335
Net cash used in investing activities	(424,267)	(273,653)	(347,258)
Financing Activities:
Issuance of long-term debt	198,000	0	310,000
Premium on issuance of long-term debt [Abstract]	0	0	31,384
Retirement of long-term debt	(4,360)	0	(175,000)
Dividends on common stock	(154,287)	(146,119)	(137,813)
Acquisition of treasury stock	(3,111)	(3,031)	(4,641)
Make-whole premium on retirement of long-term debt	0	0	(3,305)
Other	926	334	3,636
Net cash used in financing activities	35,316	(149,484)	16,989
Net (decrease) increase in cash and cash equivalents	(37,666)	(59,873)	57,862
Cash and cash equivalents at beginning of the year	215,243	275,116	217,254
Cash and cash equivalents at end of the year	177,577	215,243	275,116
Cash paid during the period for:
Income taxes	45,885	34,330	9,975
Interest (net of amount capitalized)	85,985	83,499	81,074
Non-cash investing activities:
Additions to property, plant and equipment in accounts payable	$ 84,324	$ 53,690	$ 45,004