CONDENSED CONSOLIDATED STATEMENTS OF INCOME Unaudited - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Operating revenue	$ 218,983	$ 211,731	$ 523,862	$ 504,899
Operations:
Water production costs	78,818	75,261	191,797	181,460
Administrative and general	26,493	22,749	77,195	66,908
Other operations	21,943	21,208	60,307	55,660
Maintenance	6,768	6,057	17,596	16,877
Depreciation and amortization	21,009	19,231	62,677	57,650
Income taxes	11,262	18,219	15,380	27,941
Property and other taxes	7,142	6,544	20,253	18,717
Total operating (income) expenses	173,435	169,269	445,205	425,213
Net operating income	45,548	42,462	78,657	79,686
Other income and expenses:
Non-regulated revenue	4,703	3,542	13,967	10,743
Non-regulated expenses	(4,897)	(2,576)	(16,449)	(6,244)
Other components of net periodic benefit cost	(1,975)	(2,137)	(6,984)	(7,023)
Allowance for equity funds used during construction	1,023	1,105	2,644	2,763
Income tax benefit (expense) on other income and expenses	305	30	1,882	(1,105)
Net other loss	(841)	(36)	(4,940)	(866)
Interest expense:
Interest expense	10,875	9,284	30,207	27,073
Allowance for borrowed funds used during construction	(560)	(707)	(1,359)	(1,765)
Net interest expense	10,315	8,577	28,848	25,308
Net income	$ 34,392	$ 33,849	$ 44,869	$ 53,512
Earnings per share:
Basic (in dollars per share)	$ 0.72	$ 0.70	$ 0.93	$ 1.11
Diluted (in dollars per share)	$ 0.72	$ 0.70	$ 0.93	$ 1.11
Weighted average shares outstanding:
Basic (in shares)	48,070	48,017	48,058	48,007
Diluted (in shares)	48,070	48,017	48,058	48,007
Dividends declared per share of common stock (in dollars per share)	$ 0.1875	$ 0.18	$ 0.5625	$ 0.5400