Condensed Financials Standalone Holdco Condensed Financials Standalone Holdco (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Operations and maintenance	$ 204,227	$ 180,129	$ 178,191
Total operating expenses	697,283	625,328	602,898
Income from operations	163,117	148,351	143,474
Deferred income taxes	14,617	18,667	6,317
Other income (expense), net	(12,559)	(13,944)	(22,836)
Interest expense, net	44,486	43,052	42,685
Income before income taxes	106,072	91,355	77,953
Income Tax Expense (Benefit)	27,406	21,082	12,642
Net income	78,666	76,781	61,735
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	18,559	30,168
Income taxes receivable	0	6,000
Other current assets	59,288	56,150
Total current assets	437,276	323,245
Other investments	89,278	83,743
Other non-current assets	56,757	49,129
Total non-current assets	3,627,328	3,433,134
Total assets	4,064,604	3,756,379
Short-term debt	389,500	304,525
Accounts payable	133,486	97,966
Taxes accrued	15,520	13,812
Other current liabilities	54,432	52,330
Interest accrued	7,503	7,441
Total current liabilities	724,765	627,083
Long-term debt	(1,044,587)	(860,081)
Total deferred credits and other non-current liabilities	1,360,106	1,380,482
Common stock - no par value	590,771	565,112
Retained earnings	355,779	336,523
Total equity	935,146	888,733
Total liabilities and equity	4,064,604	3,756,379
Statement of Cash Flows [Abstract]
Net income	78,666	76,781	61,735
Deferred income taxes	14,617	18,667	6,317
Other	(11,569)	6,942	(14,760)
Receivables, net	44,128	16,799	(5,844)
Income and other taxes	3,292	(10,710)	4,528
Accounts payable	12,118	(15,910)	(16,485)
Additional Cash Flow Elements and Supplemental Cash Flow Information [Abstract]
Other, net	(26,069)	(2,925)	(4,080)
Cash provided by operating activities	160,353	145,318	186,013
Cash used in investing activities	(300,123)	(294,277)	(303,781)
Proceeds from Issuance of Common Stock	17,501	0	92,956
Changes in other short-term debt, net	280,000	(39,600)	(68,520)
Cash dividend payments on common stock	(55,919)	(55,420)	(53,339)
Other	(5,121)	(3,228)	(599)
Cash provided by (used in) financing activities	131,436	171,777	115,498
Cash, cash equivalents and restricted cash, beginning of period	30,168
Cash, cash equivalents and restricted cash, end of period	18,559	30,168
Total Dividends Paid to Registrant, Consolidated Subsidiaries	82,100	74,400	88,400
Comprehensive income (loss)	80,164	74,612	59,556
Parent Company, Condensed Statement of Cash Flows [Member]
Income Statement [Abstract]
Equity in earnings of subsidiaries, net of tax	83,072	78,450	64,328
Deferred income taxes	(212)	20	(198)
Net income	78,666	76,781	61,735
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	265	11,267	119
Statement of Cash Flows [Abstract]
Net income	78,666	76,781	61,735
Equity in earnings of subsidiaries, net of tax	83,072	78,450	64,328
SEC Schedule, 12-04, Cash Dividends Paid to Registrant, Consolidated Subsidiaries	56,057	55,387	53,439
Deferred income taxes	(212)	20	(198)
Other	119	65	66
Receivables, net	12,558	(12,788)	846
Income and other taxes	1,299	(7,451)	4,325
Accounts payable	3,342	8,809	(5,177)
Additional Cash Flow Elements and Supplemental Cash Flow Information [Abstract]
Increase (Decrease) in Interest Payable, Net	57	77	(32)
Other, net	313	364	346
Cash provided by operating activities	68,501	42,086	50,330
Contributions to subsidiaries	(142,405)	(47,194)	(157,591)
Proceeds from Equity Method Investment, Distribution, Return of Capital	26,000	19,000	35,000
Cash used in investing activities	(116,405)	(28,194)	(122,591)
Proceeds from Issuance of Common Stock	17,501	0	92,956
Changes in other short-term debt, net	71,000	49,000	24,000
Cash dividend payments on common stock	(55,919)	(55,420)	(53,339)
Other	4,320	3,676	4,752
Cash provided by (used in) financing activities	36,902	(2,744)	68,369
Increase (decrease) in cash and cash equivalents	(11,002)	11,148	(3,892)
Cash, cash equivalents and restricted cash, beginning of period	11,267	119	4,011
Cash, cash equivalents and restricted cash, end of period	265	11,267	119
Cash Dividends Paid to Registrant, Consolidated Subsidiaries, Gross			88,400
Parent Company, Condensed Balance Sheet [Member]
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	265	11,267
Receivables from affiliates	2,180	14,738
Income taxes receivable	0	6,000
Other current assets	11,348	6,223
Total current assets	13,793	38,228
Investments in subsidiaries	1,080,949	936,184
Other investments	42	17
Deferred Income Tax Assets, Net	383	171
Other non-current assets	613	213
Total non-current assets	1,081,987	936,585
Total assets	1,095,780	974,813
Short-term debt	144,000	73,000
Accounts payable	286	119
Payables to affiliates	16,105	12,912
Other current liabilities	243	49
Total current liabilities	160,634	86,080
Total equity	935,146	888,733
Total liabilities and equity	1,095,780	974,813
Additional Cash Flow Elements and Supplemental Cash Flow Information [Abstract]
Cash, cash equivalents and restricted cash, beginning of period	11,267
Cash, cash equivalents and restricted cash, end of period	265	11,267
Parent Company, Condensed Income Statement [Member]
Income Statement [Abstract]
Operations and maintenance	4,837	771	2,747
Total operating expenses	4,837	771	2,747
Income from operations	(4,837)	(771)	(2,747)
Equity in earnings of subsidiaries, net of tax	(83,072)	(78,450)	(64,328)
Other income (expense), net	(143)	57	(22)
Interest expense, net	982	1,557	726
Income before income taxes	77,110	76,179	60,833
Income Tax Expense (Benefit)	(1,556)	(602)	(902)
Net income	78,666	76,781	61,735
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	1,498	(2,169)	(2,179)
Statement of Cash Flows [Abstract]
Net income	78,666	76,781	61,735
Equity in earnings of subsidiaries, net of tax	(83,072)	(78,450)	(64,328)
Additional Cash Flow Elements and Supplemental Cash Flow Information [Abstract]
Comprehensive income (loss)	$ 80,164	74,612	$ 59,556
Adjustment to Stockholders' Equity Attributable to Parent		$ (3,600)