Consolidated Statments of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net income	$ 67,181	$ 48,675	$ 45,017
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	78,592	65,203	63,182
Amortization of debt premium and expenses	920	871	825
Changes in normalized deferred income taxes	21,087	26,818	24,393
Change in value of life insurance contracts	(3,058)	(1,026)	218
Allowance for equity funds used during construction	(3,750)	0	0
Stock-based compensation	3,118	2,849	2,578
(Gain) loss on sale of non-utility property	(663)	146	(315)
Write-off of capital costs	1,293	3,221	0
Changes in operating assets and liabilities:
Receivables	(31,871)	(343)	1,855
Unbilled revenue	(4,528)	(2,047)	559
Taxes, prepaid expenses, and other assets	(3,718)	1,276	(2,366)
Accounts payable	1,564	3,839	(819)
Other current liabilities	2,164	4,056	(1,106)
Other changes in noncurrent assets and liabilities	19,511	6,906	10,948
Net cash provided by operating activities	147,842	160,444	144,969
Investing activities:
Utility plant expenditures	(259,194)	(228,938)	(176,833)
Proceeds from sale of non-utility assets	666	395	319
TCP settlement proceeds	56,004	0	0
Life insurance benefits	1,558	495	0
Purchase of life insurance	(5,605)	(2,857)	(2,032)
Change in restricted cash	(81)	66	288
Net cash used in investing activities	(206,652)	(230,839)	(178,258)
Financing activities:
Short-term borrowings, net of expenses of $0 for 2017 and 2016, and $1,197 for 2015	265,000	145,100	94,303
Repayment of short-term borrowings	(87,000)	(81,615)	(141,000)
Issuance of long-term debt, net of expenses of $0 for 2017, $177 for 2016, and $707 for 2015	0	49,823	99,343
Advances and contributions in aid of construction	21,369	21,448	16,026
Refunds of advances for construction	(8,378)	(6,885)	(6,726)
Retirement of long-term debt	(26,829)	(6,996)	(7,003)
Repurchase of common stock	(1,505)	(744)	(338)
Dividends paid	(34,563)	(33,081)	(32,066)
Net cash provided by financing activities	128,094	87,050	22,539
Change in cash and cash equivalents	69,284	16,655	(10,750)
Cash and cash equivalents at beginning of year	25,492	8,837	19,587
Cash and cash equivalents at end of year	94,776	25,492	8,837
Cash paid (received) during the year for:
Interest (net of amounts capitalized)	32,223	28,038	25,345
Income tax refunds	(1,697)	0	0
Supplemental disclosure of investing and financing non-cash activities:
Accrued payables for investments in utility plant	41,017	27,150	21,546
Utility plant contributed by developers	19,898	16,824	7,383
Litigation proceeds for MTBE contamination reclassified from other long-term liabilities to depreciable plant and equipment	$ 2,420	$ 484	$ 332