STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net income	$ 159,213	$ 152,157	$ 127,263
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	193,870	162,957	153,241
Demand side management program amortization	1,673	1,673	1,673
Deferred income taxes	126,465	122,983	62,836
Amortization of investment tax credits	(133)	(213)	(213)
Allowance for equity funds used during construction	(9,310)	(9,981)	(7,378)
Provision for bad debts	5,091	6,066	4,655
Net derivative losses	63	217	268
Other	(28)	122	(3,827)
Changes in operating assets and liabilities:
Accounts receivable	(10,392)	(8,868)	(3,291)
Accrued unbilled revenues	(10,386)	(15,637)	25,506
Inventories	(1,928)	(959)	5,686
Prepayments and other	4,267	22,651	(24,712)
Accounts payable	11,836	13,776	(24,570)
Net regulatory assets and liabilities	38,137	(55,689)	26,452
Other current liabilities	3,427	5,156	(30,762)
Pension and other employee benefit obligations	(21,679)	(15,276)	(9,405)
Change in other noncurrent assets	(1,206)	(200)	2,352
Change in other noncurrent liabilities	(18,524)	6,748	8,974
Net cash provided by operating activities	470,456	387,683	314,748
Investing activities
Utility capital/construction expenditures	(559,865)	(512,522)	(599,511)
Allowance for equity funds used during construction	9,310	9,981	7,378
Proceeds from insurance recoveries	0	3,901	0
Investments in utility money pool arrangement	(142,000)	(75,000)	(92,000)
Receipts from utility money pool arrangement	77,000	75,000	92,000
Other	(493)	(1,174)	3,136
Net cash used in investing activities	(616,048)	(499,814)	(588,997)
Financing activities
(Repayment of) proceeds from short-term borrowings, net	(50,000)	35,000	(22,000)
Proceeds from issuance of long-term debt	442,338	295,985	198,496
Repayment of long-term debt, including reacquisition premiums	(271,613)	(200,000)	0
Borrowings under utility money pool arrangement	335,000	636,500	579,700
Repayments under utility money pool arrangement	(335,000)	(636,500)	(595,700)
Capital contributions from parent	143,659	66,225	214,535
Dividends paid to parent	(108,765)	(85,069)	(100,544)
Net cash provided by financing activities	155,619	112,141	274,487
Net change in cash and cash equivalents	10,027	10	238
Cash and cash equivalents at beginning of period	844	834	596
Cash and cash equivalents at end of period	10,871	844	834
Supplemental disclosure of cash flow information:
Cash paid for interest (net of amounts capitalized)	(75,978)	(78,236)	(76,474)
Cash paid for income taxes, net	41,548	61,813	(23,987)
Supplemental disclosure of non-cash investing transactions:
Property, plant and equipment additions in accounts payable	$ 77,563	$ 43,074	$ 44,335