Segment Information (Company's Segment Information, Continuing Operations) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating revenues	$ 226,042	$ 243,626	$ 218,892	$ 201,132	$ 205,747	$ 226,137	$ 211,860	$ 194,347	$ 889,692	$ 838,091	$ 809,525
Operations and maintenance expense	85,321	82,022	86,445	79,314	92,393	68,624	73,515	73,946	333,102	308,478	282,253
Depreciation									158,179	146,032	136,302
Amortization									(1,703)	641	422
Operating income (loss)	85,301	106,470	80,949	67,439	62,794	104,293	86,754	69,337	340,159	323,178	333,920
Interest expense, net									125,383	99,054	88,543
Interest income									25,406	152	202
Allowance for funds used during construction									16,172	13,023	15,211
Equity earnings in joint venture									(2,210)	(2,081)	(331)
Provision for income taxes (benefit)									(13,017)	(13,669)	16,914
Net income (loss)	64,227	$ 88,489	$ 54,903	$ 16,924	(3,657)	$ 78,216	$ 66,590	$ 50,839	224,543	191,988	239,738
Capital expenditures									550,273	495,737	478,089
Total assets	9,361,985				6,964,496				9,361,985	6,964,496	6,332,463
Regulated [Member]
Segment Reporting Information [Line Items]
Operating revenues									886,430	834,638	804,905
Operations and maintenance expense									315,052	292,232	282,009
Depreciation									158,162	145,977	136,246
Amortization									(2,264)	401	240
Operating income (loss)									357,979	338,388	331,888
Interest expense, net									97,995	89,207	82,102
Interest income									54	95	128
Allowance for funds used during construction									16,172	13,023	15,211
Provision for income taxes (benefit)									(1,267)	4,158	14,107
Net income (loss)									274,920	259,160	246,548
Capital expenditures									550,273	495,730	478,077
Total assets	7,269,404				6,807,960				7,269,404	6,807,960	6,236,109
Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues									3,262	3,453	4,620
Operations and maintenance expense									18,050	16,246	244
Depreciation									17	55	56
Amortization									561	240	182
Operating income (loss)									(17,820)	(15,210)	2,032
Interest expense, net									27,388	9,847	6,441
Interest income									25,352	57	74
Equity earnings in joint venture									(2,210)	(2,081)	(331)
Provision for income taxes (benefit)									(11,750)	(17,827)	2,807
Net income (loss)									(50,377)	(67,172)	(6,810)
Capital expenditures										7	12
Total assets	$ 2,092,581				$ 156,536				$ 2,092,581	$ 156,536	$ 96,354