CONDENSED CONSOLIDATED STATEMENTS OF INCOME Unaudited - USD ($) shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Operating revenue	$ 232,537			$ 221,288			$ 537,679	$ 530,779
Operations:
Water production costs	80,568			78,818			190,795	191,797
Administrative and general	26,779			26,493			81,310	77,195
Other operations	24,550			21,943			64,913	60,307
Maintenance	7,065			6,768			19,212	17,596
Depreciation and amortization	22,273			21,009			66,967	62,677
Income taxes	12,194			11,786			13,524	16,950
Property and other taxes	7,541			7,142			21,902	20,253
Total operating (income) expenses	180,970			173,959			458,623	446,775
Net operating income	51,567			47,329			79,056	84,004
Other income and expenses:
Non-regulated revenue	4,118			4,703			14,149	13,967
Non-regulated expenses	(4,351)			(4,897)			(10,470)	(16,449)
Other components of net periodic benefit cost	(1,857)			(1,975)			(4,308)	(6,984)
Allowance for equity funds used during construction	1,868			1,023			5,087	2,644
Income tax benefit on other income and expenses	330			305			(985)	1,882
Net other income (loss)	108			(841)			3,473	(4,940)
Interest expense:
Interest expense	10,279			10,875			33,532	30,207
Allowance for borrowed funds used during construction	(1,028)			(560)			(2,783)	(1,359)
Net interest expense	9,251			10,315			30,749	28,848
Net income	$ 42,424	$ 16,996	$ (7,640)	$ 36,173	$ 14,805	$ (762)	$ 51,780	$ 50,216
Earnings per share:
Basic (in dollars per share)	$ 0.88			$ 0.75			$ 1.08	$ 1.04
Diluted (in dollars per share)	$ 0.88			$ 0.75			$ 1.08	$ 1.04
Weighted average shares outstanding:
Basic (in shares)	48,141			48,070			48,121	48,058
Diluted (in shares)	48,141			48,070			48,121	48,058