Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Cash flows from operating activities
Net income	$ 243,028	$ 248,056	$ 199,714
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation of property, plant and equipment	256,069	246,158	238,114
Other amortization	38,772	32,544	52,664
Provision for credit losses	2,037	(25,825)	50,811
Loans originated, held for sale	(128,195)	(340,986)	(564,525)
Proceeds from sale of loans, held for sale	136,504	364,848	567,652
Gain on sale of investment securities, net	(8,123)	(528)	(9,275)
Gain on sale of loans	(1,692)	(9,305)	(23,734)
Deferred income taxes	(32,502)	(5,398)	(1,706)
Share-based compensation expense	10,366	9,135	5,810
Allowance for equity funds used during construction	(10,574)	(9,534)	(8,768)
Other	(7,580)	(7,060)	1,366
Changes in assets and liabilities
Decrease (increase) in accounts receivable and unbilled revenues, net	(151,551)	(73,811)	2,533
Decrease (increase) in fuel oil stock	(87,569)	(45,819)	34,202
Decrease (increase) in regulatory assets	34,600	(13,874)	1,007
Increase (decrease) in regulatory liabilities	44,888	15,358	(16,562)
Increase (decrease) in accounts, interest and dividends payable	28,169	9,925	(20,068)
Change in prepaid and accrued income taxes, tax credits and utility revenue taxes	121,636	41,331	(35,610)
Decrease in defined benefit pension and other postretirement benefit plans liability	(5,191)	(6,660)	(2,029)
Change in other assets and liabilities	(28,616)	(52,882)	(42,189)
Net cash provided by operating activities	454,476	375,673	429,407
Cash flows from investing activities
Available-for-sale investment securities purchased	(366,177)	(1,464,644)	(1,361,594)
Principal repayments on available-for-sale investment securities	342,354	583,238	478,351
Proceeds from sale of available-for-sale investment securities	0	197,354	169,157
Purchases of held-to-maturity investment securities	0	(349,579)	(146,738)
Proceeds from repayments or maturities of held-to-maturity investment securities	29,432	53,654	59,894
Purchase of stock from Federal Home Loan Bank	(173,768)	(33,022)	(27,350)
Redemption of stock from Federal Home Loan Bank	157,208	31,702	27,104
Net decrease (increase) in loans held for investment	(661,492)	72,489	(229,311)
Proceeds from sale of residential loans	0	59,844	0
Purchase of loans held for investment	(102,504)	0	0
Proceeds from sale of real estate held for sale	3,806	0	0
Capital expenditures	(344,037)	(314,524)	(383,895)
Proceeds from sale of low income housing investments	0	0	6,725
Contributions to low income housing investments	(844)	(16,131)	(9,403)
Acquisition of business	(25,706)	0	0
Other, net	13,046	27	3,412
Net cash used in investing activities	(1,128,682)	(1,179,592)	(1,413,648)
Cash flows from financing activities
Net increase (decrease) in deposit liabilities	(2,516)	785,255	1,115,055
Net decrease in short-term borrowings with original maturities of three months or less	83,652	(10,493)	(71,219)
Proceeds from issuance of short-term debt	35,000	0	165,000
Repayment of short-term debt	0	(65,000)	(150,000)
Net increase (decrease) in other bank borrowings with original maturities of three months or less	606,815	(1,365)	(25,440)
Proceeds from issuance of other bank borrowings	0	0	30,000
Repayment of other bank borrowings	0	0	(30,000)
Proceeds from issuance of long-term debt	227,312	285,886	415,997
Repayment of long-term debt and funds transferred for repayment of long-term debt	(221,910)	(82,262)	(178,969)
Withheld shares for employee taxes on vested share-based compensation	(3,165)	(2,006)	(5,700)
Common stock dividends	(153,229)	(148,643)	(144,096)
Preferred stock dividends of subsidiaries	(1,890)	(1,890)	(1,890)
Other	(2,398)	(3,080)	(3,203)
Net cash used in financing activities	567,671	756,402	1,115,535
Net increase (decrease) in cash and equivalents	(106,535)	(47,517)	131,294
Cash, cash equivalents and restricted cash, January 1	311,462	358,979	227,685
Cash, cash equivalents and restricted cash, December 31	204,927	311,462	358,979
Less: Restricted cash	(5,050)	(5,911)	(17,558)
Cash and cash equivalents, December 31	$ 199,877	$ 305,551	$ 341,421