Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Operating revenue	$ 150,930	$ 184,268	$ 152,445	$ 121,727	$ 138,426	$ 183,543	$ 144,414	$ 121,985	$ 609,370	$ 588,368	$ 597,499
Operations:
Purchased water									181,515	168,557	177,884
Purchased power									27,180	27,890	33,159
Pump taxes									11,298	11,479	12,898
Administrative and general									98,474	113,110	97,373
Other									80,082	67,248	65,807
Maintenance									22,993	21,463	19,854
Depreciation and amortization									63,599	61,381	61,217
Income taxes									24,804	24,528	26,727
Property and other taxes									23,231	21,559	20,733
Total operating expenses									533,176	517,215	515,652
Net operating income	$ 21,335	$ 30,055	$ 18,534	$ 6,270	$ 14,962	$ 32,201	$ 16,493	$ 7,497	76,194	71,153	81,847
Other income and expenses:
Non-regulated revenue									16,585	15,624	17,318
Non-regulated expenses									(11,445)	(14,044)	(14,285)
(Loss) gain on sale of non-utility property									(146)	315	51
Income tax expense on other income and expenses									(2,012)	(761)	(1,245)
Net other income									2,982	1,134	1,839
Interest expense:
Interest expense									33,466	29,185	28,483
Less: capitalized interest									(2,965)	(1,915)	(1,535)
Net interest expense									30,501	27,270	26,948
Net income									$ 48,675	$ 45,017	$ 56,738
Earnings per share:
Basic (in dollars per share)									$ 1.02	$ 0.94	$ 1.19
Diluted (in dollars per share)	$ 0.31	$ 0.48	$ 0.24	$ (0.02)	$ 0.18	$ 0.52	$ 0.21	$ 0.03	$ 1.01	$ 0.94	$ 1.19
Weighted average number of common shares outstanding:
Basic (in shares)									47,953	47,865	47,791
Diluted (in shares)									47,956	47,880	47,829