Condensed Financials Standalone Holdco Condensed Financials Standalone Holdco (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Operations and maintenance		$ 180,129	$ 178,191	$ 156,698
Total operating expenses		625,328	602,898	573,981
Income from operations		148,351	143,474	132,162
Deferred income taxes		18,667	6,317	14,356
Other income (expense), net		(13,944)	(22,836)	(3,601)
Interest expense, net		43,052	42,685	37,059
Income before income taxes		91,355	77,953	91,502
Income Tax Expense (Benefit)		21,082	12,642	24,191
Net income		76,781	61,735	64,569
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	$ 12,633	9,648	12,633	12,633	$ 30,168	$ 9,648
Income taxes receivable					6,000	256
Other current assets					44,741	38,004
Total current assets					323,245	293,692
Other investments					49,259	63,333
Other non-current assets					49,129	38,464
Total non-current assets					3,433,134	3,134,762
Total assets					3,756,379	3,428,454
Short-term debt					304,525	149,100
Accounts payable					97,966	113,370
Taxes accrued					13,812	11,971
Other current liabilities					52,330	62,705
Interest accrued					7,441	7,451
Total current liabilities					627,083	482,173
Long-term debt					(860,081)	(805,955)
Total deferred credits and other non-current liabilities					1,380,482	1,274,327
Common stock - no par value					565,112	558,282
Retained earnings					336,523	318,450
Total equity					888,733	865,999
Total liabilities and equity					3,756,379	3,428,454
Statement of Cash Flows [Abstract]
Net income		76,781	61,735	64,569
Deferred income taxes		5,005	5,445	11,453
Other		6,942	(13,907)	(1,596)
Receivables, net		16,799	(5,844)	(181)
Income and other taxes		(10,710)	4,528	(16,904)
Accounts payable		(15,910)	(16,485)	16,792
Other, net		(12,467)	(4,140)	(3,419)
Cash provided by operating activities		143,020	185,298	168,771
Cash used in investing activities		(294,277)	(303,781)	(217,453)
Proceeds from stock options exercised		68	2,015	1,546
Proceeds from Issuance of Common Stock		0	92,956	0
Changes in other short-term debt, net		(39,600)	(68,520)	163,274
Cash dividend payments on common stock		(55,420)	(53,339)	(51,311)
Other		(3,296)	(2,614)	(715)
Cash provided by (used in) financing activities		171,777	115,498	57,843
Cash and cash equivalents, beginning of period		9,648	12,633	3,472
Cash and cash equivalents, end of period	12,633	30,168	9,648	12,633
Total Dividends Paid to Registrant, Consolidated Subsidiaries	0	74,400	88,400
Parent Company, Condensed Statement of Cash Flows [Member]
Income Statement [Abstract]
Equity in earnings of subsidiaries, net of tax	36,469	78,450	64,328
Deferred income taxes	7	20	(198)
Net income	35,839	76,781	61,735
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	4,011	11,267	4,011	4,011	11,267	119
Statement of Cash Flows [Abstract]
Net income	35,839	76,781	61,735
Equity in earnings of subsidiaries, net of tax	36,469	78,450	64,328
Other	15	65	66
Receivables, net	(585)	(12,788)	846
Income and other taxes	(9,034)	(7,451)	4,325
Accounts payable	9,304	8,809	(5,177)
Increase (Decrease) in Interest Payable, Net	32	77	(32)
Other, net	44	364	346
Cash provided by operating activities	(935)	42,086	50,330
Contributions to subsidiaries	(1,804)	(47,194)	(157,591)
Proceeds from Equity Method Investment, Distribution, Return of Capital	0	19,000	35,000
Cash used in investing activities	(1,804)	(28,194)	(122,591)
Proceeds from stock options exercised	0	68	2,015
Proceeds from Issuance of Common Stock	0	0	92,956
Changes in other short-term debt, net	0	49,000	24,000
SEC Schedule, 12-04, Cash Dividends Paid to Registrant, Consolidated Subsidiaries	0	55,387	53,439
Cash dividend payments on common stock	(12,923)	(55,420)	(53,339)
Capital contributions	20,000	0	0
Other	(327)	3,608	2,737
Cash provided by (used in) financing activities	6,750	(2,744)	68,369
Increase (decrease) in cash and cash equivalents	4,011	11,148	(3,892)
Cash and cash equivalents, beginning of period		119	4,011	0
Cash and cash equivalents, end of period	4,011	11,267	119	4,011
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		119	119		11,267	119
Receivables from affiliates					14,738	1,950
Income taxes receivable					6,000	256
Other current assets					6,223	4,600
Total current assets					38,228	6,925
Investments in subsidiaries					939,741	888,477
Other investments					17	24
Deferred Income Tax Assets, Net					171	191
Other non-current assets					213	245
Total non-current assets					940,142	888,937
Total assets					978,370	895,862
Short-term debt					73,000	24,000
Accounts payable					119	612
Payables to affiliates					12,912	3,697
Taxes accrued					0	127
Other current liabilities					49	37
Total current liabilities					86,080	28,473
Common stock - no par value					847,193	840,364
Retained earnings					45,097	27,025
Total equity					892,290	867,389
Total liabilities and equity					$ 978,370	$ 895,862
Statement of Cash Flows [Abstract]
Cash and cash equivalents, beginning of period		119
Cash and cash equivalents, end of period		11,267	119
Parent Company, Condensed Income Statement [Member]
Income Statement [Abstract]
Operations and maintenance		771	2,747	838
Total operating expenses		771	2,747	838
Income from operations		(771)	(2,747)	(838)
Equity in earnings of subsidiaries, net of tax	(36,500)	(78,450)	(64,328)	(36,469)
Other income (expense), net		57	(22)	36
Interest expense, net		(1,557)	(726)	(53)
Income before income taxes		76,179	60,833	35,614
Income Tax Expense (Benefit)		(602)	(902)	(225)
Net income		76,781	61,735	35,839
Statement of Cash Flows [Abstract]
Net income		76,781	61,735	35,839
Equity in earnings of subsidiaries, net of tax	$ (36,500)	$ (78,450)	$ (64,328)	$ (36,469)