Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Operating revenue	$ 714,557	$ 698,196	$ 676,113
Operations:
Purchased water	212,461	207,103	199,081
Purchased power	31,362	31,080	28,862
Pump taxes	11,518	14,664	13,924
Administrative and general	108,617	100,781	93,326
Other operations	90,061	79,868	74,448
Maintenance	26,834	24,494	22,530
Depreciation and amortization	89,220	83,781	76,783
Income taxes	16,280	18,589	35,279
Property and other taxes	28,792	27,296	24,797
Total operating expenses	615,145	587,656	569,030
Net operating income	99,412	110,540	107,083
Other income and expenses:
Non-regulated revenue	19,205	18,272	15,898
Non-regulated expenses	(13,869)	(22,787)	(9,390)
Other components of net periodic benefit cost	(5,733)	(9,308)	(9,588)
Allowance for equity funds used during construction	6,685	3,954	3,750
Gain on sale of non-utility property	28	50	663
Income tax (expense) benefit on other income and expenses	(1,391)	2,717	(1,548)
Net other income (loss)	4,925	(7,102)	(215)
Interest expense:
Interest expense	44,891	39,917	36,288
Allowance for borrowed funds used during construction	(3,670)	(2,063)	(2,360)
Net interest expense	41,221	37,854	33,928
Net income	$ 63,116	$ 65,584	$ 72,940
Earnings per share:
Basic (in dollars per share)	$ 1.31	$ 1.36	$ 1.52
Diluted (in dollars per share)	$ 1.31	$ 1.36	$ 1.52
Weighted average number of common shares outstanding:
Basic (in shares)	48,168	48,060	48,009
Diluted (in shares)	48,168	48,060	48,009