Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net income	$ 203,664	$ 167,187	$ 250,146
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation of property, plant and equipment	214,036	200,658	194,273
Other amortization	41,593	21,340	10,473
Provision for loan losses	14,745	10,901	16,763
Loans originated and purchased, held for sale	(109,537)	(115,104)	(236,769)
Proceeds from sale of loans, held for sale	112,182	127,951	236,062
Deferred income taxes	(9,368)	37,835	47,118
Share-based compensation expense	7,792	5,404	4,789
Allowance for equity funds used during construction	(10,877)	(12,483)	(8,325)
Other	(4,219)	(3,324)	(12,422)
Changes in assets and liabilities
Increase in accounts receivable and unbilled revenues, net	(64,321)	(12,875)	(898)
Decrease (increase) in fuel oil stock	7,054	(20,794)	4,786
Decrease (increase) in regulatory assets	9,252	(17,256)	(18,273)
Increase (decrease) in accounts, interest and dividends payable	21,528	34,985	(9,643)
Change in prepaid and accrued income taxes, tax credits and utility revenue taxes	29,429	20,685	39,109
Increase in defined benefit pension and other postretirement benefit plans liability	20,871	882	1,587
Change in other assets and liabilities	15,488	(25,551)	(23,118)
Net cash provided by operating activities	499,312	420,441	495,658
Cash flows from investing activities
Available-for-sale investment securities purchased	(224,335)	(528,379)	(533,956)
Principal repayments on available-for-sale investment securities	218,930	220,231	219,845
Proceeds from sale of available-for-sale investment securities	0	0	16,423
Purchases of held-to-maturity investment securities	(103,184)	(44,515)	0
Proceeds from repayments or maturities of held-to-maturity investment securities	5,720	0	0
Purchase of stock from Federal Home Loan Bank	(28,292)	(2,868)	(7,773)
Redemption of stock from Federal Home Loan Bank	28,040	4,380	7,233
Net decrease (increase) in loans held for investment	(189,352)	15,887	(194,042)
Proceeds from sale of commercial loans	7,149	36,760	52,299
Proceeds from sale of real estate acquired in settlement of loans	589	1,019	829
Proceeds from sale of real estate held for sale	0	0	1,764
Capital expenditures	(537,369)	(495,187)	(330,043)
Contributions in aid of construction	30,599	64,733	30,100
Contributions to low income housing investments	(14,499)	(17,505)	0
Acquisition of business	0	(76,323)	0
Other, net	13,945	6,468	856
Net cash used in investing activities	(792,059)	(815,299)	(736,465)
Cash flows from financing activities
Net increase in deposit liabilities	165,880	341,668	523,675
Net increase (decrease) in short-term borrowings with original maturities of three months or less	(18,999)	67,992	(103,063)
Proceeds from issuance of short-term debt	25,000	125,000	0
Repayment of short-term debt	(50,000)	(75,000)	0
Net increase (decrease) in retail repurchase agreements	26,556	61,776	(43,601)
Proceeds from other bank borrowings	696,000	59,500	180,835
Repayments of other bank borrowings	(701,000)	(123,034)	(272,902)
Proceeds from issuance of long-term debt	250,000	532,325	115,000
Repayment of long-term debt and funds transferred for redemption of special purpose revenue bonds	(53,887)	(465,000)	(75,000)
Withheld shares for employee taxes on vested share-based compensation	(996)	(3,828)	(2,416)
Net proceeds from issuance of common stock	0	0	13,220
Common stock dividends	(134,987)	(134,873)	(117,274)
Preferred stock dividends of subsidiaries	(1,890)	(1,890)	(1,890)
Other	(1,603)	(6,349)	2,197
Net cash used in financing activities	200,074	378,287	218,781
Net decrease in cash and equivalents	(92,673)	(16,571)	(22,026)
Cash and cash equivalents, January 1	261,881	278,452	300,478
Cash and cash equivalents, December 31	$ 169,208	$ 261,881	$ 278,452