Consolidated Statements Of Net Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Consolidated Statements Of Net Income [Abstract]
Operating revenues	$ 889,692	$ 838,091	$ 809,525
Operating expenses:
Operations and maintenance	333,102	308,478	282,253
Depreciation	158,179	146,032	136,302
Amortization	(1,703)	641	422
Taxes other than income taxes	59,955	59,762	56,628
Total operating expenses	549,533	514,913	475,605
Operating income	340,159	323,178	333,920
Other expense (income):
Interest expense	125,383	99,054	88,543
Interest income	(25,406)	(152)	(202)
Allowance for funds used during construction	(16,172)	(13,023)	(15,211)
Change in fair value of interest rate swap agreements	23,742	59,779
Loss on debt extinguishment	18,528
Gain on sale of other assets	(923)	(714)	(484)
Equity earnings in joint venture	(2,210)	(2,081)	(331)
Other	5,691	1,996	4,953
Income before income taxes	211,526	178,319	256,652
Provision for income taxes (benefit)	(13,017)	(13,669)	16,914
Net income	$ 224,543	$ 191,988	$ 239,738
Net income per common share:
Basic	$ 1.04	$ 1.08	$ 1.35
Diluted	$ 1.04	$ 1.08	$ 1.35
Average common shares outstanding during the period:
Basic	215,550	177,904	177,612
Diluted	215,931	178,399	178,175