Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Operating Activities:
Net Income	$ 261,893	$ 259,560	$ 245,872
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	199,908	173,555	179,444
Deferred income taxes and investment tax credits	39,613	(511)	23,901
Changes in regulatory assets and liabilities	(4,748)	(79,693)	(33,705)
Pension and postretirement benefit plan expense	27,155	29,286	33,817
Contributions to pension and postretirement benefit plans	(55,337)	(44,192)	(44,220)
Earnings of unconsolidated equity-method investments	12,426	11,511	11,435
Distributions from unconsolidated equity-method investments	2,950	11,586	11,711
Allowance for equity funds used during construction	(43,221)	(37,285)	(31,537)
Other non-cash adjustments to net income, net	8,414	14,892	8,929
Change in:
Accounts receivable	(17,628)	(81,545)	(9,434)
Accounts payable and other accrued liabilities	(3,220)	(2,156)	(6,581)
Taxes accrued/receivable	(53,243)	(11,626)	991
Other current assets	(81,244)	112,602	17,700
Other current liabilities	(12,551)	(4,628)	(17,885)
Other assets	10,712	22,951	(4,304)
Net cash provided by operating activities	267,027	351,285	363,264
Investing Activities:
Additions to property, plant and equipment	(611,137)	(432,589)	(299,999)
Payments received from transmission project joint funding partners	26,501	17,778	5,876
Payments to Acquire Partners Interest in Real Estate Partnership, Net of Cash Acquired	(2,533)	(9,881)	(15,148)
Proceeds from Equity Method Investment, Distribution, Return of Capital	0	8,489	14,439
Payments to Acquire Equity Securities, FV-NI	(12,235)	(45,572)	(17,186)
Debt Securities, Held-to-Maturity, Purchase	(1,617)	(31,224)	0
Proceeds from Sale of Debt and Equity Securities, FV-NI, Held-for-investment	8,921	63,857	11,328
Payments to Acquire Short-term Investments	0	25,000	25,000
Proceeds from Maturities, Prepayments and Calls of Short-term Investments	0	25,000	50,000
Other	2,153	4,875	2,037
Net cash used in investing activities	(589,947)	(424,267)	(273,653)
Financing Activities:
Issuance of long-term debt	872,000	198,000	0
Premium on issuance of long-term debt [Abstract]	(7,006)	0	0
Retirement of long-term debt	(225,000)	(4,360)	0
Dividends on common stock	(163,545)	(154,287)	(146,119)
Acquisition of treasury stock	(3,274)	(3,111)	(3,031)
Other	(403)	(926)	(334)
Net cash used in financing activities	472,772	35,316	(149,484)
Net increase (decrease) in cash and cash equivalents	149,852	(37,666)	(59,873)
Cash and cash equivalents at beginning of the year	177,577	215,243	275,116
Cash and cash equivalents at end of the year	327,429	177,577	215,243
Cash paid during the period for:
Income taxes	6,200	45,885	34,330
Interest (net of amount capitalized)	97,742	85,985	83,499
Non-cash investing activities:
Additions to property, plant and equipment in accounts payable	$ 185,400	$ 84,324	$ 53,690