Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Operating revenue	$ 167,417	$ 221,288	$ 174,938	$ 134,553	$ 164,296	$ 214,037	$ 173,438	$ 124,342	$ 698,196	$ 676,113	$ 609,370
Operations:
Purchased water									207,103	199,081	181,515
Purchased power									31,080	28,862	27,180
Pump taxes									14,664	13,924	11,298
Administrative and general									100,781	93,326	87,601
Other operations									79,868	74,448	80,082
Maintenance									24,494	22,530	22,993
Depreciation and amortization									83,781	76,783	63,599
Income taxes									18,589	35,279	29,235
Property and other taxes									27,296	24,797	23,231
Total operating expenses									587,656	569,030	526,734
Net operating income	$ 26,536	$ 47,329	$ 26,839	$ 9,836	$ 23,077	$ 43,902	$ 28,111	$ 11,993	110,540	107,083	82,636
Other income and expenses:
Non-regulated revenue									18,272	15,898	16,585
Non-regulated expenses									(22,787)	(9,390)	(11,445)
Other components of net periodic benefit cost									(9,308)	(9,588)	(10,873)
Allowance for equity funds used during construction									3,954	3,750	0
Gain (loss) on sale of non-utility property									50	663	(146)
Income tax benefit (expense) on other income and expenses									2,717	(1,548)	2,419
Net other loss									(7,102)	(215)	(3,460)
Interest expense:
Interest expense									39,917	36,288	33,466
Allowance for borrowed funds used during construction									(2,063)	(2,360)	(2,965)
Net interest expense									37,854	33,928	30,501
Net income									$ 65,584	$ 72,940	$ 48,675
Earnings per share:
Basic (in dollars per share)									$ 1.36	$ 1.52	$ 1.02
Diluted (in dollars per share)	$ 0.32	$ 0.75	$ 0.31	$ (0.02)	$ 0.31	$ 0.74	$ 0.42	$ 0.05	$ 1.36	$ 1.52	$ 1.01
Weighted average number of common shares outstanding:
Basic (in shares)									48,060	48,009	47,953
Diluted (in shares)									48,060	48,009	47,956