Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Operating revenue	$ 794,307	$ 714,557	$ 698,196
Operations:
Purchased water	230,076	212,461	207,103
Purchased power	34,006	31,362	31,080
Pump taxes	12,647	11,518	14,664
Administrative and general	117,058	108,617	100,781
Other operations	95,859	90,061	79,868
Maintenance	28,026	26,834	24,494
Depreciation and amortization	98,505	89,220	83,781
Income taxes	11,435	16,280	18,589
Property and other taxes	30,029	28,792	27,296
Total operating expenses	657,641	615,145	587,656
Net operating income	136,666	99,412	110,540
Other income and expenses:
Non-regulated revenue	16,922	19,205	18,272
Non-regulated expenses	(14,300)	(13,869)	(22,787)
Other components of net periodic benefit cost	(4,988)	(5,733)	(9,308)
Allowance for equity funds used during construction	4,976	6,685	3,954
Gain on sale of non-utility property	0	28	50
Income tax (expense) benefit on other income and expenses	(583)	(1,391)	2,717
Net other income (loss)	2,027	4,925	(7,102)
Interest expense:
Interest expense	45,047	44,891	39,917
Allowance for borrowed funds used during construction	(3,185)	(3,670)	(2,063)
Net interest expense	41,862	41,221	37,854
Net income	$ 96,831	$ 63,116	$ 65,584
Earnings per share:
Basic (in dollars per share)	$ 1.97	$ 1.31	$ 1.36
Diluted (in dollars per share)	$ 1.97	$ 1.31	$ 1.36
Weighted average number of common shares outstanding:
Basic (in shares)	49,274	48,168	48,060
Diluted (in shares)	49,274	48,168	48,060