CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net income	$ 463,491	$ 466,802	$ 455,188
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	446,179	416,427	383,992
Demand side management program amortization	2,138	3,509	4,331
Deferred income taxes	222,002	277,896	227,823
Amortization of Investment Tax Credits	(2,805)	(2,807)	(2,941)
Allowance for equity funds used during construction	(18,557)	(14,485)	(46,784)
Provision for bad debts	14,121	13,052	17,005
Net realized and unrealized hedging and derivative transactions	1,325	2,414	(2,578)
Other Operating Activities, Cash Flow Statement	(388)	2,500	0
Changes in operating assets and liabilities:
Accounts receivable	(14,227)	8,872	(42,921)
Accrued unbilled revenues	(20,866)	17,837	(23,132)
Inventories	172	33,417	(972)
Prepayments and other	68,693	10,483	(81,715)
Accounts payable	38,439	(40,982)	(22,789)
Net regulatory assets and liabilities	4,143	78,055	130,499
Other current liabilities	1,892	19,654	5,284
Pension and other employee benefit obligations	(10,627)	(23,449)	(38,905)
Change in other noncurrent assets	(6,750)	4,086	5,537
Change in other noncurrent liabilities	(22,120)	(35,334)	(19,130)
Net cash provided by (used in) operating activities	1,166,255	1,237,947	947,792
Investing activities
Utility capital/construction expenditures	(1,113,800)	(995,597)	(1,114,338)
Proceeds from Insurance Settlement, Investing Activities	608	0	0
Allowance for equity funds used during construction	18,557	14,485	46,784
Investments in utility money pool arrangement	(444,000)	(196,300)	(603,000)
Repayments from utility money pool arrangement	444,000	212,300	659,000
Payments for (Proceeds from) Other Investing Activities	(1,460)	0	0
Net cash provided by (used in) investing activities	(1,096,095)	(965,112)	(1,011,554)
Financing activities
Proceeds from (repayments of) short-term borrowings, net	115,000	(368,000)	382,000
Borrowings under utility money pool arrangement	524,500	165,000	333,000
Repayments under utility money pool arrangement	(524,500)	(165,000)	(333,000)
Proceeds from issuance of long-term debt	244,507	246,751	295,598
Repayments of long-term debt	(129,500)	0	(275,000)
Capital contributions from parent	38,755	175,210	81,498
Dividends paid to parent	(336,581)	(330,846)	(433,788)
Net cash provided by (used in) financing activities	(67,819)	(276,885)	50,308
Net change in cash and cash equivalents	2,341	(4,050)	(13,454)
Cash and cash equivalents at beginning of period	3,585	7,635	21,089
Cash and cash equivalents at end of period	5,926	3,585	7,635
Supplemental disclosure of cash flow information:
Cash paid for interest (net of amounts capitalized)	(171,714)	(165,546)	(150,011)
Cash received (paid) for income taxes, net	22,827	13,822	(91,810)
Supplemental disclosure of non-cash investing transactions:
Property, plant and equipment additions in accounts payable	$ 68,870	$ 106,912	$ 139,616