Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Operating revenue	$ 161,991	$ 211,731	$ 171,132	$ 122,036	$ 150,930	$ 184,268	$ 152,445	$ 121,727	$ 666,890	$ 609,370	$ 588,368
Operations:
Purchased water									199,081	181,515	168,557
Purchased power									28,862	27,180	27,890
Pump taxes									13,924	11,298	11,479
Administrative and general									102,914	98,474	113,110
Other operations									74,448	80,082	67,248
Maintenance									22,530	22,993	21,463
Depreciation and amortization									76,783	63,599	61,381
Income taxes									28,928	24,804	24,528
Property and other taxes									24,797	23,231	21,559
Total operating expenses									572,267	533,176	517,215
Net operating income	$ 20,118	$ 41,196	$ 25,259	$ 8,050	$ 21,335	$ 30,055	$ 18,534	$ 6,270	94,623	76,194	71,153
Other income and expenses:
Non-regulated revenue									15,898	16,585	15,624
Non-regulated expenses									(9,390)	(11,445)	(14,044)
Allowance for equity funds used during construction									3,750	0	0
Gain (loss) on sale of non-utility property									663	(146)	315
Income tax expense on other income and expenses									(4,435)	(2,012)	(761)
Net other income									6,486	2,982	1,134
Interest expense:
Interest expense									36,288	33,466	29,185
Allowance for borrowed funds used during construction									(2,360)	(2,965)	(1,915)
Net interest expense									33,928	30,501	27,270
Net income									$ 67,181	$ 48,675	$ 45,017
Earnings per share:
Basic (in dollars per share)									$ 1.40	$ 1.02	$ 0.94
Diluted (in dollars per share)	$ 0.29	$ 0.70	$ 0.39	$ 0.02	$ 0.31	$ 0.48	$ 0.24	$ (0.02)	$ 1.40	$ 1.01	$ 0.94
Weighted average number of common shares outstanding:
Basic (in shares)									48,009	47,953	47,865
Diluted (in shares)									48,009	47,956	47,880