Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net income	$ 219,772,000	$ 203,664,000	$ 167,187,000
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation of property, plant and equipment	229,858,000	214,036,000	200,658,000
Other amortization	48,255,000	41,593,000	21,340,000
Provision for loan losses	23,480,000	14,745,000	10,901,000
Loans originated, held for sale	(285,042,000)	(109,537,000)	(115,104,000)
Proceeds from sale of loans, held for sale	277,119,000	112,182,000	127,951,000
Gain on sale of real estate, held for sale	(10,762,000)	0	0
Deferred income taxes	(15,085,000)	(9,368,000)	37,835,000
Share-based compensation expense	9,986,000	7,792,000	5,404,000
Allowance for equity funds used during construction	(11,987,000)	(10,877,000)	(12,483,000)
Other	10,822,000	(4,219,000)	(3,324,000)
Changes in assets and liabilities
Decrease (increase) in accounts receivable and unbilled revenues, net	26,083,000	(64,321,000)	(12,875,000)
Decrease (increase) in fuel oil stock	(11,493,000)	7,054,000	(20,794,000)
Decrease (increase) in regulatory assets	71,262,000	9,252,000	(17,256,000)
Increase (decrease) in accounts, interest and dividends payable	(3,054,000)	21,528,000	34,985,000
Change in prepaid and accrued income taxes, tax credits and utility revenue taxes	(27,538,000)	29,429,000	20,685,000
Increase (decrease) in defined benefit pension and other postretirement benefit plans liability	(4,482,000)	20,871,000	882,000
Change in other assets and liabilities	(34,724,000)	15,488,000	(25,551,000)
Net cash provided by operating activities	512,470,000	499,312,000	420,441,000
Cash flows from investing activities
Available-for-sale investment securities purchased	(108,088,000)	(224,335,000)	(528,379,000)
Principal repayments on available-for-sale investment securities	272,949,000	218,930,000	220,231,000
Proceeds from sale of available-for-sale investment securities	19,810,000	0	0
Purchases of held-to-maturity investment securities	(13,057,000)	(103,184,000)	(44,515,000)
Proceeds from repayments or maturities of held-to-maturity investment securities	15,505,000	5,720,000	0
Purchase of stock from Federal Home Loan Bank	(95,636,000)	(28,292,000)	(2,868,000)
Redemption of stock from Federal Home Loan Bank	97,160,000	28,040,000	4,380,000
Net decrease (increase) in loans held for investment	(300,210,000)	(189,352,000)	15,887,000
Proceeds from sale of commercial loans	0	7,149,000	36,760,000
Proceeds from sale of real estate held for sale	21,060,000	0	0
Capital expenditures	(457,520,000)	(506,770,000)	(430,454,000)
Contributions to low income housing investments	(6,974,000)	(14,499,000)	(17,505,000)
Acquisition of business	0	0	(76,323,000)
Other, net	13,292,000	14,534,000	7,487,000
Net cash used in investing activities	(541,709,000)	(792,059,000)	(815,299,000)
Cash flows from financing activities
Net increase in deposit liabilities	113,050,000	165,880,000	341,668,000
Net increase (decrease) in short-term borrowings with original maturities of three months or less	86,718,000	(18,999,000)	67,992,000
Proceeds from issuance of short-term debt	75,000,000	25,000,000	125,000,000
Repayment of short-term debt	(50,000,000)	(50,000,000)	(75,000,000)
Net increase in other bank borrowings with original maturities of three months or less	5,070,000	71,556,000	61,776,000
Repayment of other bank borrowings	0	(50,000,000)	(63,534,000)
Proceeds from issuance of long-term debt	289,349,000	250,000,000	532,325,000
Repayment of long-term debt and funds transferred for repayment of long-term debt	(287,285,000)	(53,887,000)	(465,000,000)
Withheld shares for employee taxes on vested share-based compensation	(997,000)	(996,000)	(3,828,000)
Common stock dividends	(139,463,000)	(134,987,000)	(134,873,000)
Preferred stock dividends of subsidiaries	(1,890,000)	(1,890,000)	(1,890,000)
Other	(1,836,000)	(1,603,000)	(6,349,000)
Net cash used in financing activities	87,716,000	200,074,000	378,287,000
Net decrease in cash and equivalents	58,477,000	(92,673,000)	(16,571,000)
Cash, cash equivalents and restricted cash, January 1	169,208,000	261,881,000	278,452,000
Cash, cash equivalents and restricted cash, December 31	227,685,000	169,208,000	261,881,000
Less: Restricted cash	(30,872,000)	0	0
Cash and cash equivalents, December 31	$ 196,813,000	$ 169,208,000	$ 261,881,000