CONDENSED CONSOLIDATED STATEMENTS OF INCOME Unaudited - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Operating revenue	$ 211,731	$ 184,268	$ 504,899	$ 458,440
Operations:
Water production costs	75,261	70,175	181,460	168,833
Administrative and general	24,886	23,844	73,931	75,037
Other operations	21,208	19,561	55,660	57,766
Maintenance	6,057	5,545	16,877	17,542
Depreciation and amortization	19,231	15,884	57,650	47,772
Income taxes	17,348	13,247	26,099	19,192
Property and other taxes	6,544	5,957	18,717	17,439
Total operating (income) expense	170,535	154,213	430,394	403,581
Net operating income (loss)	41,196	30,055	74,505	54,859
Other income and expenses:
Non-regulated revenue	3,542	3,397	10,743	10,589
Non-regulated expenses	(2,576)	(2,517)	(6,244)	(8,306)
Allowance for equity funds used during construction	1,105	0	2,763	0
Income tax expense on other income and expenses	(841)	(349)	(2,947)	(914)
Net other income	1,230	531	4,315	1,369
Interest expense:
Interest expense	9,284	8,485	27,073	24,984
Allowance for borrowed funds used during construction	(707)	(774)	(1,765)	(2,341)
Net interest expense	8,577	7,711	25,308	22,643
Net income	$ 33,849	$ 22,875	$ 53,512	$ 33,585
Earnings per share:
Basic (in dollars per share)	$ 0.70	$ 0.48	$ 1.11	$ 0.70
Diluted (in dollars per share)	$ 0.70	$ 0.48	$ 1.11	$ 0.70
Weighted average shares outstanding:
Basic (in shares)	48,017	47,969	48,007	47,949
Diluted (in shares)	48,017	47,969	48,007	47,952
Dividends declared per share of common stock (in dollars per share)	$ 0.1800	$ 0.1725	$ 0.5400	$ 0.5175