Consolidated Statements Of Operations And Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Consolidated Statements Of Net Income [Abstract]
Operating revenues	$ 243,626	$ 226,137	$ 663,650	$ 632,344
Operating expenses:
Operations and maintenance	82,022	68,624	247,781	216,085
Depreciation	39,489	37,457	118,113	110,037
Amortization	444	199	(2,140)	478
Taxes other than income taxes	15,201	15,564	45,038	45,360
Total operating expenses	137,156	121,844	408,792	371,960
Operating income	106,470	104,293	254,858	260,384
Other expense (income):
Interest expense	32,643	25,403	92,239	72,664
Interest income	(9,680)	(44)	(18,117)	(111)
Allowance for funds used during construction	(4,613)	(3,066)	(12,280)	(8,510)
Change in fair value of interest rate swap agreements			23,742
Loss on debt extinguishment			18,920
Gain on sale of other assets	(175)	(261)	(443)	(598)
Equity earnings in joint venture	(135)	(215)	(1,918)	(1,508)
Other	1,494	325	4,293	1,365
Income before income taxes	86,936	82,151	148,422	197,082
Provision for income taxes (benefit)	(1,553)	3,935	(11,894)	1,437
Net income	88,489	78,216	160,316	195,645
Comprehensive income	$ 88,489	$ 78,216	$ 160,316	$ 195,645
Net income per common share:
Basic	$ 0.38	$ 0.44	$ 0.76	$ 1.10
Diluted	$ 0.38	$ 0.44	$ 0.76	$ 1.10
Average common shares outstanding during the period:
Basic	232,053	177,923	209,971	177,876
Diluted	232,464	178,357	210,335	178,347