Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities
Net income	$ 248,056	$ 199,714	$ 219,772
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation of property, plant and equipment	246,158	238,114	229,858
Other amortization	32,544	52,664	48,255
Provision for credit losses	(25,825)	50,811	23,480
Loans originated, held for sale	(340,986)	(564,525)	(285,042)
Proceeds from sale of loans, held for sale	364,848	567,652	277,119
Gain on sale of real estate, held for sale	0	0	(10,762)
Gain on sale of investment securities, net	(528)	(9,275)	(653)
Gain on sale of loans	(9,305)	(23,734)	(4,943)
Deferred income taxes	(5,398)	(1,706)	(15,085)
Share-based compensation expense	9,135	5,810	9,986
Allowance for equity funds used during construction	(9,534)	(8,768)	(11,987)
Other	(7,060)	1,366	18,568
Changes in assets and liabilities
Decrease (increase) in accounts receivable and unbilled revenues, net	(73,811)	2,533	23,933
Decrease (increase) in fuel oil stock	(45,819)	34,202	(11,493)
Decrease (increase) in regulatory assets	(13,874)	1,007	71,262
Increase (decrease) in regulatory liabilities	15,358	(16,562)	1,953
Increase (decrease) in accounts, interest and dividends payable	9,925	(20,068)	(3,054)
Change in prepaid and accrued income taxes, tax credits and utility revenue taxes	41,331	(35,610)	(27,538)
Decrease in defined benefit pension and other postretirement benefit plans liability	(6,660)	(2,029)	(4,482)
Change in other assets and liabilities	(52,882)	(42,189)	(36,677)
Net cash provided by operating activities	375,673	429,407	512,470
Cash flows from investing activities
Available-for-sale investment securities purchased	(1,464,644)	(1,361,594)	(108,088)
Principal repayments on available-for-sale investment securities	583,238	478,351	272,949
Proceeds from sale of available-for-sale investment securities	197,354	169,157	19,810
Purchases of held-to-maturity investment securities	(349,579)	(146,738)	(13,057)
Proceeds from repayments or maturities of held-to-maturity investment securities	53,654	59,894	15,505
Purchase of stock from Federal Home Loan Bank	(33,022)	(27,350)	(95,636)
Redemption of stock from Federal Home Loan Bank	31,702	27,104	97,160
Net decrease (increase) in loans held for investment	72,489	(229,311)	(300,210)
Proceeds from sale of residential loans	59,844	0	0
Proceeds from sale of real estate held for sale	0	0	21,060
Capital expenditures	(314,524)	(383,895)	(457,520)
Proceeds from sale of low income housing investments	0	6,725	1
Contributions to low income housing investments	(16,131)	(9,403)	(6,974)
Other, net	27	3,412	13,291
Net cash used in investing activities	(1,179,592)	(1,413,648)	(541,709)
Cash flows from financing activities
Net increase in deposit liabilities	785,255	1,115,055	113,050
Net increase (decrease) in short-term borrowings with original maturities of three months or less	(10,493)	(71,219)	86,718
Proceeds from issuance of short-term debt	0	165,000	75,000
Repayment of short-term debt	(65,000)	(150,000)	(50,000)
Net increase (decrease) in other bank borrowings with original maturities of three months or less	(1,365)	(25,440)	5,070
Proceeds from issuance of other bank borrowings	0	30,000	0
Repayment of other bank borrowings	0	(30,000)	0
Proceeds from issuance of long-term debt	285,886	415,997	289,349
Repayment of long-term debt and funds transferred for repayment of long-term debt	(82,262)	(178,969)	(287,285)
Withheld shares for employee taxes on vested share-based compensation	(2,006)	(5,700)	(997)
Common stock dividends	(148,643)	(144,096)	(139,463)
Preferred stock dividends of subsidiaries	(1,890)	(1,890)	(1,890)
Other	(3,080)	(3,203)	(1,836)
Net cash used in financing activities	756,402	1,115,535	87,716
Net increase (decrease) in cash and equivalents	(47,517)	131,294	58,477
Cash, cash equivalents and restricted cash, January 1	358,979	227,685	169,208
Cash, cash equivalents and restricted cash, December 31	311,462	358,979	227,685
Less: Restricted cash	(5,911)	(17,558)	(30,872)
Cash and cash equivalents, December 31	$ 305,551	$ 341,421	$ 196,813