CONDENSED CONSOLIDATED STATEMENTS OF INCOME Unaudited - USD ($) shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Operating revenue	$ 179,031		$ 174,938		$ 305,142	$ 309,491
Operations:
Water production costs	64,635		65,373		110,227	112,979
Administrative and general	25,434		24,383		54,531	50,702
Other operations	22,542		20,724		40,363	38,364
Maintenance	5,692		5,389		12,147	10,828
Depreciation and amortization	22,326		20,953		44,694	41,668
Income taxes	4,321		4,870		1,330	5,164
Property and other taxes	7,068		6,407		14,361	13,111
Total operating (income) expenses	152,018		148,099		277,653	272,816
Net operating income	27,013		26,839		27,489	36,675
Other income and expenses:
Non-regulated revenue	5,130		4,845		10,031	9,264
Non-regulated expenses	(3,900)		(6,115)		(6,119)	(11,552)
Other components of net periodic benefit cost	(1,192)		(2,463)		(2,451)	(5,009)
Allowance for equity funds used during construction	1,686		710		3,219	1,621
Income tax (expense) benefit on other income and expenses	(487)		819		(1,315)	1,577
Net other income (loss)	1,237		(2,204)		3,365	(4,099)
Interest expense:
Interest expense	12,178		10,134		23,253	19,332
Allowance for borrowed funds used during construction	(924)		(304)		(1,755)	(799)
Net interest expense	11,254		9,830		21,498	18,533
Net income (loss)	$ 16,996	$ (7,640)	$ 14,805	$ (762)	$ 9,356	$ 14,043
Earnings per share:
Basic (in dollars per share)	$ 0.35		$ 0.31		$ 0.19	$ 0.29
Diluted (in dollars per share)	$ 0.35		$ 0.31		$ 0.19	$ 0.29
Weighted average shares outstanding:
Basic (in shares)	48,136		48,073		48,111	48,051
Diluted (in shares)	48,136		48,073		48,111	48,051