Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Operating revenue	$ 138,426	$ 183,543	$ 144,414	$ 121,985	$ 137,384	$ 191,184	$ 158,416	$ 110,515	$ 588,368	$ 597,499	$ 584,103
Operations:
Purchased water									168,557	177,884	183,046
Purchased power									27,890	33,159	32,220
Pump taxes									11,479	12,898	10,795
Administrative and general									113,110	97,373	98,055
Other									67,248	65,807	69,738
Maintenance									21,463	19,854	17,368
Depreciation and amortization									61,381	61,217	58,320
Income taxes									24,528	26,727	19,047
Property and other taxes									21,559	20,733	21,509
Total operating expenses									517,215	515,652	510,098
Net operating income	$ 14,962	$ 32,201	$ 16,493	$ 7,497	$ 16,569	$ 40,834	$ 23,292	$ 1,152	71,153	81,847	74,005
Other income and expenses:
Non-regulated revenue									15,624	17,318	14,795
Non-regulated expense									(14,044)	(14,285)	(11,265)
Gain on sale of non-utility property									315	51	0
Income tax (expense) on other income and expenses									(761)	(1,245)	(1,422)
Net other income									1,134	1,839	2,108
Interest expense:
Interest expense									29,185	28,483	30,897
Less: capitalized interest									(1,915)	(1,535)	(2,038)
Net interest expense									27,270	26,948	28,859
Net income									$ 45,017	$ 56,738	$ 47,254
Earnings per share:
Basic (in dollars per share)									$ 0.94	$ 1.19	$ 1.02
Diluted (in dollars per share)	$ 0.18	$ 0.52	$ 0.21	$ 0.03	$ 0.24	$ 0.70	$ 0.36	$ (0.11)	$ 0.94	$ 1.19	$ 1.02
Weighted average number of common shares outstanding:
Basic (in shares)									47,865	47,791	46,384
Diluted (in shares)									47,880	47,829	46,417