Condensed Financials Standalone Holdco Condensed Financials Standalone Holdco (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Operations and maintenance									$ 156,698	$ 152,358	$ 136,723
Total operating expenses									573,981	604,136	517,456
Income from operations									132,162	150,902	150,717
Other income (expense), net									(3,601)	(295)	(7,151)
Interest expense, net									37,059	37,526	38,136
Income before income taxes									91,502	113,081	105,430
Income Tax Expense (Benefit)									24,191	41,008	43,011
Net income (loss)	$ 35,824	$ (11,794)	$ (998)	$ 41,537	$ (90,167)	$ (8,495)	$ 2,729	$ 40,310	64,569	(55,623)	58,895
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	12,633			3,472	3,472			3,521	3,472	3,521	4,211	$ 12,633	$ 3,472
Income taxes receivable												6,000	0
Other current assets												28,472	24,949
Total current assets												295,921	269,936
Other investments												63,558	66,363
Other non-current assets												14,149	6,505
Total non-current assets												2,946,741	2,769,810
Total assets												3,242,662	3,039,746
Accounts payable												115,878	111,021
Interest accrued												7,306	6,773
Total current liabilities												509,084	381,850
Long-term debt												(706,247)	(683,184)
Deferred tax liabilities												280,463	270,526
Total deferred credits and other non-current liabilities												1,264,697	1,231,936
Common stock - no par value; authorized 100,000 shares; issued and outstanding 28,880 and 28,736 at December 31, 2018 and 2017, respectively												457,640	448,865
Retained earnings												312,182	302,349
Total equity												762,634	742,776
Total liabilities and equity												3,242,662	3,039,746
Statement of Cash Flows [Abstract]
Net income (loss)	35,824	$ (11,794)	$ (998)	41,537	(90,167)	$ (8,495)	$ 2,729	40,310	64,569	(55,623)	58,895
Deferred income taxes									11,453	15,700	29,853
Other									(1,596)	(2,102)	(2,846)
Receivables, net									(181)	(3,282)	6,395
Income and other taxes									(16,904)	6,734	9,467
Accounts payable									16,792	1,092	12,028
Interest accrued									526	807	93
Other, net									552	(4,093)	11,727
Cash provided by operating activities									168,771	206,704	222,147
Cash used in investing activities									(217,453)	(214,172)	(136,629)
Cash dividend payments on common stock									(51,311)	(53,957)	(51,508)
Other									(715)	(2,309)	(3,087)
Cash provided by (used in) financing activities									57,843	7,419	(86,208)
Increase (decrease) in cash and cash equivalents									9,161	(49)	(690)
Cash and cash equivalents, beginning of period				3,472				$ 3,521	3,472	3,521	4,211
Cash and cash equivalents, end of period	12,633				3,472				12,633	3,472	$ 3,521
Parent Company, Condensed Statement of Cash Flows [Member]
Income Statement [Abstract]
Equity in earnings of subsidiaries, net of tax									(36,469)
Net income (loss)									35,839
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	4,011			0	0				0	0		4,011	$ 0
Statement of Cash Flows [Abstract]
Net income (loss)									35,839
Equity in earnings of subsidiaries, net of tax									(36,469)
Deferred income taxes									7
Other									15
Receivables, net									(585)
Income and other taxes									(9,034)
Accounts payable									9,304
Interest accrued									32
Other, net									44
Cash provided by operating activities									(935)
Contributions to subsidiaries									(1,804)
Cash used in investing activities									(1,804)
Cash dividend payments on common stock									(12,923)
Capital contributions									20,000
Other									(327)
Cash provided by (used in) financing activities									6,750
Increase (decrease) in cash and cash equivalents									4,011
Cash and cash equivalents, beginning of period				$ 0					0
Cash and cash equivalents, end of period	4,011				$ 0				4,011	$ 0
Parent Company, Condensed Balance Sheet [Member]
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	4,011								4,011			4,011
Receivables from affiliates												2,796
Income taxes receivable												6,000
Other current assets												3,078
Total current assets												15,885
Investments in subsidiaries												754,971
Other investments												65
Other non-current assets												310
Total non-current assets												755,346
Total assets												771,231
Accounts payable												168
Payables to affiliates												9,166
Interest accrued												32
Total current liabilities												9,366
Long-term debt												(1)
Deferred tax liabilities												7
Total deferred credits and other non-current liabilities												7
Common stock - no par value; authorized 100,000 shares; issued and outstanding 28,880 and 28,736 at December 31, 2018 and 2017, respectively												739,722
Retained earnings												22,137
Total equity												761,859
Total liabilities and equity												$ 771,231
Statement of Cash Flows [Abstract]
Cash and cash equivalents, end of period	$ 4,011								4,011
Parent Company, Condensed Income Statement [Member]
Income Statement [Abstract]
Operations and maintenance									838
Total operating expenses									838
Income from operations									(838)
Equity in earnings of subsidiaries, net of tax									(36,469)
Other income (expense), net									36
Interest expense, net									53
Income before income taxes									35,614
Income Tax Expense (Benefit)									(225)
Net income (loss)									35,839
Statement of Cash Flows [Abstract]
Net income (loss)									35,839
Equity in earnings of subsidiaries, net of tax									$ (36,469)