CONDENSED CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Operating revenue	$ 304,108			$ 232,537			$ 605,155	$ 537,679
Operations:
Water production costs	85,344			80,568			210,462	190,795
Administrative and general	29,208			26,779			85,827	81,310
Other operations	29,746			24,550			69,618	64,913
Maintenance	7,129			7,065			20,924	19,212
Depreciation and amortization	24,699			22,273			73,733	66,967
Income taxes	13,804			12,194			10,489	13,524
Property and other taxes	8,116			7,541			22,470	21,902
Total operating (income) expenses	198,046			180,970			493,523	458,623
Net operating income	106,062			51,567			111,632	79,056
Other income and expenses:
Non-regulated revenue	3,934			4,118			11,969	14,149
Non-regulated expenses	(2,865)			(4,351)			(11,811)	(10,470)
Other components of net periodic benefit cost	(1,008)			(1,857)			(3,770)	(4,308)
Allowance for equity funds used during construction	973			1,868			4,292	5,087
Income tax (expense) benefit on other income and expenses	(245)			330			(152)	(985)
Net other income	789			108			528	3,473
Interest expense:
Interest expense	11,162			10,279			33,573	33,532
Allowance for borrowed funds used during construction	(671)			(1,028)			(2,747)	(2,783)
Net interest expense	10,491			9,251			30,826	30,749
Net income	$ 96,360	$ 5,281	$ (20,307)	$ 42,424	$ 16,996	$ (7,640)	$ 81,334	$ 51,780
Earnings per share:
Basic (in dollars per share)	$ 1.94			$ 0.88			$ 1.66	$ 1.08
Diluted (in dollars per share)	$ 1.94			$ 0.88			$ 1.66	$ 1.08
Weighted average shares outstanding:
Basic (in shares)	49,576			48,141			49,034	48,121
Diluted (in shares)	49,576			48,141			49,034	48,121