Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Cash flows from operating activities
Net income	$ 199,714	$ 219,772	$ 203,664
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation of property, plant and equipment	238,114	229,858	214,036
Other amortization	52,664	48,255	41,593
Provision for credit losses	50,811	23,480	14,745
Loans originated, held for sale	(564,525)	(285,042)	(109,537)
Proceeds from sale of loans, held for sale	567,652	277,119	112,182
Gain on sale of real estate, held for sale	0	(10,762)	0
Gain on sale of investment securities, net	(9,275)	(653)	0
Gain on sale of loans	(23,734)	(4,943)	(1,493)
Deferred income taxes	(1,706)	(15,085)	(9,368)
Share-based compensation expense	5,810	9,986	7,792
Allowance for equity funds used during construction	(8,768)	(11,987)	(10,877)
Other	1,366	18,568	(521)
Changes in assets and liabilities
Decrease (increase) in accounts receivable and unbilled revenues, net	2,533	23,933	(66,526)
Decrease (increase) in fuel oil stock	34,202	(11,493)	7,054
Decrease in regulatory assets	1,007	71,262	9,252
Increase (decrease) in regulatory liabilities	(16,562)	1,953	37,358
Increase (decrease) in accounts, interest and dividends payable	(20,068)	(3,054)	21,528
Change in prepaid and accrued income taxes, tax credits and utility revenue taxes	(35,610)	(27,538)	29,429
Increase (decrease) in defined benefit pension and other postretirement benefit plans liability	(2,029)	(4,482)	20,871
Change in other assets and liabilities	(42,189)	(36,677)	(21,870)
Net cash provided by operating activities	429,407	512,470	499,312
Cash flows from investing activities
Available-for-sale investment securities purchased	(1,361,594)	(108,088)	(224,335)
Principal repayments on available-for-sale investment securities	478,351	272,949	218,930
Proceeds from sale of available-for-sale investment securities	169,157	19,810	0
Purchases of held-to-maturity investment securities	(146,738)	(13,057)	(103,184)
Proceeds from repayments or maturities of held-to-maturity investment securities	59,894	15,505	5,720
Purchase of stock from Federal Home Loan Bank	(27,350)	(95,636)	(28,292)
Redemption of stock from Federal Home Loan Bank	27,104	97,160	28,040
Net increase in loans held for investment	(229,311)	(300,210)	(189,352)
Proceeds from sale of commercial loans	0	0	7,149
Proceeds from sale of real estate held for sale	0	21,060	0
Capital expenditures	(383,895)	(457,520)	(506,770)
Proceeds from sale of low income housing investments	6,725	1	473
Contributions to low income housing investments	(9,403)	(6,974)	(14,499)
Other, net	3,412	13,291	14,061
Net cash used in investing activities	(1,413,648)	(541,709)	(792,059)
Cash flows from financing activities
Net increase in deposit liabilities	1,115,055	113,050	165,880
Net increase (decrease) in short-term borrowings with original maturities of three months or less	(71,219)	86,718	(18,999)
Proceeds from issuance of short-term debt	165,000	75,000	25,000
Repayment of short-term debt	(150,000)	(50,000)	(50,000)
Net increase (decrease) in other bank borrowings with original maturities of three months or less	(25,440)	5,070	71,556
Proceeds from issuance of other bank borrowings	30,000	0	0
Repayment of other bank borrowings	(30,000)	0	(50,000)
Proceeds from issuance of long-term debt	415,997	289,349	250,000
Repayment of long-term debt and funds transferred for repayment of long-term debt	(178,969)	(287,285)	(53,887)
Withheld shares for employee taxes on vested share-based compensation	(5,700)	(997)	(996)
Common stock dividends	(144,096)	(139,463)	(134,987)
Preferred stock dividends of subsidiaries	(1,890)	(1,890)	(1,890)
Other	(3,203)	(1,836)	(1,603)
Net cash used in financing activities	1,115,535	87,716	200,074
Net decrease in cash and equivalents	131,294	58,477	(92,673)
Cash, cash equivalents and restricted cash, January 1	227,685	169,208	261,881
Cash, cash equivalents and restricted cash, December 31	358,979	227,685	169,208
Less: Restricted cash	(17,558)	(30,872)	0
Cash and cash equivalents, December 31	$ 341,421	$ 196,813	$ 169,208